Add documentation as a Docker service and landing page

[justb4]

Idea is to add the documentation, possibly as the landing "home" page for the testbed server.

mkdocs seems best option, also to maintain (mainly markdown/.md files).

Bij adding as a Docker service similar to Traefik and pygeoapi, committing will automatically publish a new docs version.
Other option is to host on GitHub, but my pref is as a service so the server is self-contained.

There is even a choice of Themes:
https://github.com/mkdocs/mkdocs/wiki/MkDocs-Themes

[justb4]

This has been done. Next need to enhance with nice mkdocs-theme and content.