doc: create initial doc/cjdns.md for cjdns how-to documentation

Adapted in part from the CJDNS description in #23077 by Vasil Dimov
and from CJDNS documentation and feedback by Caleb James DeLisle.

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

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+# CJDNS support in Bitcoin Core
+
+It is possible to run Bitcoin Core over CJDNS, an encrypted IPv6 network that
+uses public-key cryptography for address allocation and a distributed hash table
+for routing.
+
+## What is CJDNS?
+
+CJDNS is like a distributed, shared VPN with multiple entry points where every
+participant can reach any other participant. All participants use addresses from
+the `fc00::/8` network (reserved IPv6 range). Installation and configuration is
+done outside of Bitcoin Core, similarly to a VPN (either in the host/OS or on
+the network router).
+
+Compared to IPv4/IPv6, CJDNS provides end-to-end encryption and protects nodes
+from traffic analysis and filtering.
+
+Used with Tor and I2P, CJDNS is a complementary option that can enhance network
+redundancy and robustness for both the Bitcoin network and individual nodes.
+
+Each network has different characteristics. For instance, Tor is widely used but
+somewhat centralized. I2P connections have a source address and I2P is slow.
+CJDNS is fast but does not hide the sender and the recipient from intermediate
+routers.
+
+## Installing CJDNS and connecting to the network
+
+To install and set up CJDNS, follow the instructions at
+https://github.com/cjdelisle/cjdns#cjdns.
+
+Don't skip steps
+["2. Find a friend"](https://github.com/cjdelisle/cjdns#2-find-a-friend) and
+["3. Connect your node to your friend's
+node"](https://github.com/cjdelisle/cjdns#3-connect-your-node-to-your-friends-node).
+You need to be connected to the CJDNS network before it will work with your
+Bitcoin Core node.
+
+Typically, CJDNS might be launched from its directory with
+`sudo ./cjdroute < cjdroute.conf` and it sheds permissions after setting up the
+[TUN](https://en.wikipedia.org/wiki/TUN/TAP) interface. You may also [launch it as an
+unprivileged user](https://github.com/cjdelisle/cjdns/blob/master/doc/non-root-user.md)
+with some additional setup.
+
+The network connection can be checked by running `./tools/peerStats` from the
+CJDNS directory.
+
+## Run Bitcoin Core with CJDNS
+
+Once you are connected to the CJDNS network, the following Bitcoin Core
+configuration option makes CJDNS peers automatically reachable:
+
+```
+-cjdnsreachable
+```
+
+When enabled, this option tells Bitcoin Core that it is running in an
+environment where a connection to an `fc00::/8` address will be to the CJDNS
+network instead of to an [RFC4193](https://datatracker.ietf.org/doc/html/rfc4193)
+IPv6 local network. This helps Bitcoin Core perform better address management:
+  - Your node can consider incoming `fc00::/8` connections to be from the CJDNS
+    network rather than from an IPv6 private one.
+  - If one of your node's local addresses is `fc00::/8`, then it can choose to
+    gossip that address to peers.
+
+## Additional configuration options related to CJDNS
+
+```
+-onlynet=cjdns
+```
+
+Make automatic outbound connections only to CJDNS addresses. Inbound and manual
+connections are not affected by this option. It can be specified multiple times
+to allow multiple networks, e.g. onlynet=cjdns, onlynet=i2p, onlynet=onion.
+
+CJDNS support was added to Bitcoin Core in version 23.0 and there may be fewer
+CJDNS peers than Tor or IP ones. You can use `bitcoin-cli -addrinfo` to see the
+number of CJDNS addresses known to your node.
+
+In general, a node can be run with both an onion service and CJDNS (or any/all
+of IPv4/IPv6/onion/I2P/CJDNS), which can provide a potential fallback if one of
+the networks has issues. There are a number of ways to configure this; see
+[doc/tor.md](https://github.com/bitcoin/bitcoin/blob/master/doc/tor.md) for
+details.
+
+## CJDNS-related information in Bitcoin Core
+
+There are several ways to see your CJDNS address in Bitcoin Core:
+- in the "Local addresses" output of CLI `-netinfo`
+- in the "localaddresses" output of RPC `getnetworkinfo`
+
+To see which CJDNS peers your node is connected to, use `bitcoin-cli -netinfo 4`
+or the `getpeerinfo` RPC (i.e. `bitcoin-cli getpeerinfo`).
+
+To see which CJDNS addresses your node knows, use the `getnodeaddresses 0 cjdns`
+RPC.