The readme was getting pretty unmanageable and hard to grok. This is an attempt to simplify things by moving installation instructions from the README to a separate file. I've tried to resist the temptation to fix too much stuff while I'm here - it mostly just copies-and-pastes from one doc to the other, and changes from rst to md syntax.

Also:

* Fix wrapping in docker readme
* Clean up some docs on the docker image
* a workaround for #4554

There are two reasons this is a good thing:

 * first, it means that you don't end up with stuff kicking around your working
   copy ending up in the build image by mistake (which can upset the pip
   install process)

 * second: it means that the docker image cache is more effective, and we can
   reuse docker images when iterating on the docker stuff.

Fixes #4559

Switch the matrixdotorg/synapse:latest Docker image to use python 3

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.