Add mkdocs

[nrbnlulu]

Description

added mkdocs for a better docs experience.

TODO:

• add link to github example files
• remove table of content on rendered markdowns.
• sort tabs by the TOC provided in index.md

GitHub Issues

[codecov]

Codecov Report

Merging #2672 (d1de503) into devel (e4b1605) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            devel    #2672   +/-   ##
=======================================
  Coverage   91.19%   91.19%           
=======================================
  Files         192      192           
  Lines        7843     7843           
=======================================
  Hits         7152     7152           
  Misses        691      691           

[LecrisUT]

There's a competing PR ar #2478. I think it's helpful to dicuss the merits of one over another at #2709. I'm also putting my hat on the rink for sphinx+rtd