This patch adds a new docker directory, which contains a Dockerfile plus
some additional configuration for creating a container that runs
chasquid+dovecot+letsencrypt.
It also updates the gitlab CI pipeline to automatically build and
publish an image on each commit.
This is experimental and likely to break.
This patch adds configuration to integrate with readthedocs.org, using
mkdocs as rendering engine.
It also does minor documentation updates, to tidy some things up
(clearer titles, move some documentation from the root into docs/, etc).
