This patch replaces test/README, which was becoming a bit outdated, with
a more general description of the different tests at a high level, and
includes it in the documentation index.
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).
