doc: Add detailed info about Bitcoin Core files

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-Filename            | Description
---------------------|----------------------------------------------------------------------------------------------------------------------------
-banlist.dat         | stores the IPs/Subnets of banned nodes
-bitcoin.conf        | contains configuration settings for bitcoind or bitcoin-qt
-bitcoind.pid        | stores the process id of bitcoind while running
-blocks/blk000??.dat | block data (custom, 128 MiB per file); since 0.8.0
-blocks/rev000??.dat | block undo data (custom); since 0.8.0 (format changed since pre-0.8)
-blocks/index/*      | block index (LevelDB); since 0.8.0
-chainstate/*        | blockchain state database (LevelDB); since 0.8.0
-database/*          | BDB database environment; only used for wallet since 0.8.0; moved to wallets/ directory on new installs since 0.16.0
-db.log              | wallet database log file; moved to wallets/ directory on new installs since 0.16.0
-debug.log           | contains debug information and general logging generated by bitcoind or bitcoin-qt
-fee_estimates.dat   | stores statistics used to estimate minimum transaction fees and priorities required for confirmation; since 0.10.0
-indexes/txindex/*   | optional transaction index database (LevelDB); since 0.17.0
-mempool.dat         | dump of the mempool's transactions; since 0.14.0
-peers.dat           | peer IP address database (custom format); since 0.7.0
-wallet.dat          | personal wallet (BDB) with keys and transactions; moved to wallets/ directory on new installs since 0.16.0
-wallets/database/*  | BDB database environment; used for wallets since 0.16.0
-wallets/db.log      | wallet database log file; since 0.16.0
-wallets/wallet.dat  | personal wallet (BDB) with keys and transactions; since 0.16.0
-.cookie             | session RPC authentication cookie (written at start when cookie authentication is used, deleted on shutdown): since 0.12.0
-onion_private_key   | cached Tor hidden service private key for `-listenonion`: since 0.12.0
-guisettings.ini.bak | backup of former GUI settings after `-resetguisettings` is used
-
-Only used in pre-0.8.0
----------------------
-* blktree/*; block chain index (LevelDB); since pre-0.8, replaced by blocks/index/* in 0.8.0
-* coins/*; unspent transaction output database (LevelDB); since pre-0.8, replaced by chainstate/* in 0.8.0
-
-Only used before 0.8.0
----------------------
-* blkindex.dat: block chain index database (BDB); replaced by {chainstate/*,blocks/index/*,blocks/rev000??.dat} in 0.8.0
-* blk000?.dat: block data (custom, 2 GiB per file); replaced by blocks/blk000??.dat in 0.8.0
-
-Only used before 0.7.0
----------------------
-* addr.dat: peer IP address database (BDB); replaced by peers.dat in 0.7.0
+# Bitcoin Core file system
+
+**Contents**
+
+- [Data directory location](#data-directory-location)
+
+- [Data directory layout](#data-directory-layout)
+
+- [Multi-wallet environment](#multi-wallet-environment)
+
+- [GUI settings](#gui-settings)
+
+- [Legacy subdirectories and files](#legacy-subdirectories-and-files)
+
+- [Notes](#notes)
+
+## Data directory location
+
+The data directory is the default location where the Bitcoin Core files are stored.
+
+1. The default data directory paths for supported platforms are:
+
+Platform | Data directory path
+---------|--------------------
+Linux    | `$HOME/.bitcoin/`
+macOS    | `$HOME/Library/Application Support/Bitcoin/`
+Windows  | `%APPDATA%\Bitcoin\` <sup>[\[1\]](#note1)</sup>
+
+2. The non-default data directory path can be specified by `-datadir` option.
+
+3. All content of the data directory, except for `bitcoin.conf` file, is chain-specific. This means the actual data directory paths for non-mainnet cases differ:
+
+Chain option        | Data directory path
+--------------------|--------------------
+no option (mainnet) | *path_to_datadir*`/`
+`-testnet`          | *path_to_datadir*`/testnet3/`
+`-regtest`          | *path_to_datadir*`/regtest/`
+
+## Data directory layout
+
+Subdirectory       | File(s)               | Description
+-------------------|-----------------------|------------
+`blocks/`          |                       | Blocks directory; can be specified by `-blocksdir` option (except for `blocks/index/`)
+`blocks/index/`    | LevelDB database      | Block index; `-blocksdir` option does not affect this path
+`blocks/`          | `blkNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Actual Bitcoin blocks (in network format, dumped in raw on disk, 128 MiB per file)
+`blocks/`          | `revNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Block undo data (custom format)
+`chainstate/`      | LevelDB database      | Blockchain state (a compact representation of all currently unspent transaction outputs and some metadata about the transactions they are from)
+`indexes/txindex/` | LevelDB database      | Transaction index; *optional*, used if `-txindex=1`
+`wallets/`         |                       | [Contains wallets](#multi-wallet-environment); can be specified by `-walletdir` option; if `wallets/` subdirectory does not exist, a wallet resides in the data directory
+`./`               | `banlist.dat`         | Stores the IPs/subnets of banned nodes
+`./`               | `bitcoin.conf`        | Contains [configuration settings](bitcoin-conf.md) for `bitcoind` or `bitcoin-qt`; can be specified by `-conf` option
+`./`               | `bitcoind.pid`        | Stores the process ID (PID) of `bitcoind` or `bitcoin-qt` while running; created at start and deleted on shutdown; can be specified by `-pid` option
+`./`               | `debug.log`           | Contains debug information and general logging generated by `bitcoind` or `bitcoin-qt`; can be specified by `-debuglogfile` option
+`./`               | `fee_estimates.dat`   | Stores statistics used to estimate minimum transaction fees and priorities required for confirmation
+`./`               | `guisettings.ini.bak` | Backup of former [GUI settings](#gui-settings) after `-resetguisettings` option is used
+`./`               | `mempool.dat`         | Dump of the mempool's transactions
+`./`               | `onion_private_key`   | Cached Tor hidden service private key for `-listenonion` option
+`./`               | `peers.dat`           | Peer IP address database (custom format)
+`./`               | `.cookie`             | Session RPC authentication cookie; if used, created at start and deleted on shutdown; can be specified by `-rpccookiefile` option
+`./`               | `.lock`               | Data directory lock file
+
+## Multi-wallet environment
+
+Wallets are Berkeley DB (BDB) databases:
+
+Subdirectory | File(s)           | Description
+-------------|-------------------|------------
+`database/`  | BDB logging files | Part of BDB environment; created at start and deleted on shutdown; a user *must keep it as safe* as personal wallet `wallet.dat`
+`./`         | `db.log`          | BDB error file
+`./`         | `wallet.dat`      | Personal wallet (BDB) with keys and transactions
+`./`         | `.walletlock`     | Wallet lock file
+
+1. Each user-defined wallet named "wallet_name" resides in `wallets/wallet_name/` subdirectory.
+
+2. The default (unnamed) wallet resides in `wallets/` subdirectory; if the latter does not exist, the wallet resides in the data directory.
+
+3. A wallet database path can be specified by `-wallet` option.
+
+## GUI settings
+
+`bitcoin-qt` uses [`QSettings`](https://doc.qt.io/qt-5/qsettings.html) class; this implies platform-specific [locations where application settings are stored](https://doc.qt.io/qt-5/qsettings.html#locations-where-application-settings-are-stored).
+
+## Legacy subdirectories and files
+
+These subdirectories and files are no longer used by the Bitcoin Core:
+
+Path           | Description | Repository notes
+---------------|-------------|-----------------
+`blktree/`     | Blockchain index; replaced by `blocks/index/` in [0.8.0](https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.8.0.md#improvements) | [PR #2231](https://github.com/bitcoin/bitcoin/pull/2231), [`8fdc94cc`](https://github.com/bitcoin/bitcoin/commit/8fdc94cc8f0341e96b1edb3a5b56811c0b20bd15)
+`coins/`       | Unspent transaction output database; replaced by `chainstate/` in 0.8.0 | [PR #2231](https://github.com/bitcoin/bitcoin/pull/2231), [`8fdc94cc`](https://github.com/bitcoin/bitcoin/commit/8fdc94cc8f0341e96b1edb3a5b56811c0b20bd15)
+`blkindex.dat` | Blockchain index BDB database; replaced by {`chainstate/`, `blocks/index/`, `blocks/revNNNNN.dat`<sup>[\[2\]](#note2)</sup>} in 0.8.0 | [PR #1677](https://github.com/bitcoin/bitcoin/pull/1677)
+`blk000?.dat`  | Block data (custom format, 2 GiB per file); replaced by `blocks/blkNNNNN.dat`<sup>[\[2\]](#note2)</sup> in 0.8.0 | [PR #1677](https://github.com/bitcoin/bitcoin/pull/1677)
+`addr.dat`     | Peer IP address BDB database; replaced by `peers.dat` in [0.7.0](https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.7.0.md) | [PR #1198](https://github.com/bitcoin/bitcoin/pull/1198), [`928d3a01`](https://github.com/bitcoin/bitcoin/commit/928d3a011cc66c7f907c4d053f674ea77dc611cc)
+
+## Notes
+
+<a name="note1">1</a>. The `/` (slash, U+002F) is used as the platform-independent path component separator in this paper.
+
+<a name="note2">2</a>. `NNNNN` matches `[0-9]{5}` regex.
+