Combine related admin API docs

[aaronraimist]

Currently there is a separate file for the purge remote media API docs and the rest of the media APIs
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/purge_remote_media.rst
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/media_admin_api.md

There is also separate files for the purge room API, shutdown room API, and then the rest of the room APIs
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/purge_room.md
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/shutdown_room.md
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/rooms.md

It probably makes sense for all of the media APIs to be in one file and all of the room APIs to be in another, rather than being spread out like this.

[dklimpel]

These should be deprecated and removed:
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/purge_room.md
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/shutdown_room.md

[clokep]

Agreed! Would really appreciate someone taking a once over of those and seeing how they could be better organized. Some of them (e.g. the rooms.md file) could use a table of contents at the top -- similar to the user_admin_api.md.

[anoadragon453]

These should be deprecated and removed:
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/purge_room.md
https://github.com/matrix-org/synapse/blob/master/docs/admin_api/shutdown_room.md

This is still to be done, and will be once #8829 lands.

[anoadragon453]

#8829 has now landed. Thanks @dklimpel!