😎(docs) consider switching to mkdocs

[jmaupetit]

Purpose

Writing documentation using Restructured Text format feels really painful nowadays. Now that PyPI natively supports rendering project descriptions formatted with markdown [1] and read the docs also supports markdown formatted documentation (using MkDocs), I think it's time to switch to markdown FTW!

[1] https://dustingram.com/articles/2018/03/16/markdown-descriptions-on-pypi

Proposal

• convert *.rst documentation files to *.md
• remove Sphinx and it's dependencies (edx theme, etc.)
• add mkdocs dependency and configure it
• update documentation build & clean methods (Makefile)