Document the 'databases' homeserver config option

[anoadragon453]

This PR documents the databases config option, which allows for tentative support of sharding your Synapse data storage across multiple database backends. The feature was originally implemented in #6580.

This option is considered supported (we test it in our Sytest CI runs) but potentially not production-ready. We don't use it ourselves on matrix.org and don't know of anyone else using it in production.

Nevertheless, there are some people that are eager to try it out (after having taken the necessary database backups) and documenting the option (with the included warning) will help it be discovered.

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The added documentation is probably easiest viewed by:

• checking out this PR's branch
• running mdbook serve, and
• navigating to http://localhost:3000/usage/configuration/config_documentation.html#databases

[squahtx]

Looks good, one small suggestion