Migration to MkDocs

[giohappy]

Sphinx is a great tool, with lots of features and options, and served very well for GeoNode's documentation.
However, we feel that a simpler, modular, and lightweight tool will help the refactoring and maintenance of docs, planned for GeoNode 5.

We want to assess the migration to MkDocs, following similar paths followed by MapStore and GeoNetwork. And from GeoNewtork experience, we want to test the https://github.com/jodygarnett/translate/ tool that should help with the first (rough) step.

ReadTheDocs supports MkDocs, so the overall management of updates and publishing will still be delegated to it.