Merge #17934: doc: Use CONFIG_SITE variable instead of --prefix option

223b1ba7d90509a47ea07af46f4b9c3b8efbc9f8 doc: Use CONFIG_SITE instead of --prefix (Hennadii Stepanov)

Pull request description:

  The current examples of `--prefix=...` option usage to point `configure` script to appropriate `depends` directory is not [standard](https://www.gnu.org/prep/standards/html_node/Directory-Variables.html). This causes some [confusion](https://github.com/bitcoin/bitcoin/pull/16691) and a bit of inconvenience.

  Consider a CentOS 7 32 bit system. Packages `libdb4-devel`, `libdb4-cxx-devel`, `miniupnpc-devel` and `zeromq-devel` are unavailable from repos. After recommended build with depends:
  ```
  cd depends
  make
  cd ..
  ./autogen.sh
  ./configure --prefix=$PWD/depends/i686-pc-linux-gnu
  make
  ```
  a user is unable to `make install` compiled binaries neither locally (to `~/.local`) nor system-wide (to `/usr/local`) as `--prefix` is set already.

  Meanwhile, the standard approach with using [`config.site`](https://www.gnu.org/software/automake/manual/html_node/config_002esite.html) files allows both possibilities:

  ```
  cd depends
  make
  cd ..
  ./autogen.sh
  CONFIG_SITE=$PWD/depends/i686-pc-linux-gnu/share/config.site ./configure --prefix ~/.local
  make
  make install
  ```

  or

  ```
  CONFIG_SITE=$PWD/depends/i686-pc-linux-gnu/share/config.site ./configure
  make
  sudo make install  # install to /usr/local
  ```

  Moreover, this approach is used in [Gitian descriptors](https://github.com/bitcoin/bitcoin/tree/master/contrib/gitian-descriptors) already.

ACKs for top commit:
  practicalswift:
    ACK 223b1ba7d90509a47ea07af46f4b9c3b8efbc9f8: patch looks correct
  fanquake:
    ACK 223b1ba7d90509a47ea07af46f4b9c3b8efbc9f8

Tree-SHA512: 46d97924f0fc7e95ee4566737cf7c2ae805ca500e5c49af9aa99ecc3acede4b00329bc727a110aa1b62618dfbf5d1ca2234e736f16fbdf96d6ece5f821712f54

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