Skeleton of the documentation

@gcorti @adavis @kreps @clemenci

needs !29 (merged)

URL: https://lhcbdoc.web.cern.ch/lhcbdoc/gaussino/mimazure-docs/about/gaussino.html

This is the skeleton of the documentation following the readthedocs.io theme and Moore Configuration in here: https://gitlab.cern.ch/lhcb/Moore/-/blob/master/.gitlab-ci.yml.

Gaussino's deployment is a little bit different though:

• only protected branches and MRs with a Documentation label are considered,
• it is possible to display versions/branches (as of now only master, the rest is hidden)

• we do not need to build Gaussino in gitlab CI, but instead, python paths are added explicitly when running make -C docs html

All the files in the docs directory are automatically deployed to /eos/project/l/lhcbwebsites/www/projects/gaussino and one can view the generated documentation at https://lhcbdoc.web.cern.ch/lhcbdoc/gaussino/mimazure-docs/about/gaussino.html where mimazure-docs is the current branch.

Once it's merged https://lhcbdoc.web.cern.ch/lhcbdoc/gaussino/master/about/gaussino.html will be available as well.