Add docs for ACME setup

Co-Authored-By: anoadragon453 <1342360+anoadragon453@users.noreply.github.com>

Co-Authored-By: anoadragon453 <1342360+anoadragon453@users.noreply.github.com>

Treat an invalid .well-known the same as an absent one

... basically, carry on and fall back to SRV etc.

Caching for invalid .well-knowns

Update federation routing logic to check .well-known before SRV

Also add a Measure block around the .well-known fetch

I can't figure out what's going on with #4422 and #4436; perhaps this will help.

* update debian installation instructions

* docs PR is docs

Fix remote invite rejections not coming down sync

remove trailing ,

This was broken in PR #4405, commit 886e5acc, where we changed remote
rejections to be outliers.

The fix is to explicitly add the leave event in when we know its an out
of band invite. We can't always add the event as if the server is/was in
the room there might be more events to send down the sync than just the
leave.

Server capabilities support