Add ACME docs and link to it from README and INSTALL

MSC1711 certificates FAQ

Merge branch 'anoa/self_signed_upgrade' of github.com:matrix-org/synapse into anoa/self_signed_upgrade

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

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