- Provides a toplevel documentation structure using Sphinx
 - Adds a `doc-html` target to the main Makefile
 - Converts existing documentation to RST format
 - Add some new documentation / tutorials
 - Links the developer manual and OCaml documentation
 - Synchronized documentation on Gitlab pages

This patch is co-authored by:
 - Benjamin Canou <benjamin@canou.fr>
 - Bruno Bernardo <bernardobruno@gmail.com>
 - Pietro Abate <pietro.abate@inria.fr>