Create a documentation website

EPIC: #508

DBnomics needs a documentation website containing both user documentation and contributor documentation.

First step is to initialize the documentation. Then we will gradually move pages from the technical wiki to that new documentation. In a near future, the technical wiki will contain only information for the core team which has no use for regular users.

Fetcher authors may be redirected to https://dbnomics-fetcher-toolbox.readthedocs.io/ which contains documentation about dbnomics-fetcher-toolbox Python package as well as a "Writing a fetcher" developer guide. This external documentation is part of the contributor documentation.

Tasks

• use mkdocs with material theme
• define a CI to generate a GitLab page
• reverse-proxy it on https://db.nomics.world/docs