Allow the ability to link directly to an option in the configuration manual

[anoadragon453]

Currently we can link directly to a config option's documentation via the sample config, by linking a line on GitHub. We're soon going to cull the sample config, and only place option documentation in the config manual.

It's not currently possible to link directly to an option on that page - only a section of options. This is because we lack a header for each option, which mdbook (the tool we use to generate our documentation) would make linkable. The reason we've held off from doing this is because headers are also currently added to the table of contents sidebar. This would make the ToC sidebar unwieldy on this page (#12752 would help here).

I suggest that we just go ahead and add a header for each configuration option initially though, as arguably that functionality is really useful, and worth having an unusable sidebar in the short term. By changing instances of:

Config option: `enable_room_list_search`

to

### `enable_room_list_search`