Fix incorrect heading level

A surprising number of people are using the well-known method, and are
simply copying the example configuration. This is problematic as the
example includes an explicit port, which causes inbound federation
requests to have the HTTP Host header include the port, upsetting some
reverse proxies.

Given that, we update the well-known example to be more explicit about
the various ways you can set it up, and the consequence of using an
explict port.

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>