Document how to back up a synapse server

[matrixbot]

This issue has been migrated from #2046.

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We should give users some guidance on what they need to do to effectively back up and restore a synapse.

Off the top of my head:

• database
• media repo
• homeserver.yaml
• log config, where present
• signing key (maybe, but it's fine just to use a new one)

[LaXiS96]

It looks like all that really needs to be backed up (apart from database, configuration and keys) is the local_content media folder.
It could be the case that on restoring, remote media may not be redownloaded by synapse, in that case it should be possible to use the Media Admin API to purge all remote references from the database (see https://element-hq.github.io/synapse/latest/admin_api/media_admin_api.html#purge-remote-media-api).