Automated version-control documentation for 2.1.9 and up

[SamGuay]

• Removing portray and using mike (+ mkdocs) to version-control our docs.
• GHA was modified to build and keep each version (release x.x.x version or master for dev version)

[codecov-commenter]

Codecov Report

Merging #231 (25404cf) into master (26840f3) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #231   +/-   ##
=======================================
  Coverage   78.05%   78.05%           
=======================================
  Files          15       15           
  Lines         966      966           
  Branches      197      197           
=======================================
  Hits          754      754           
  Misses        170      170           
  Partials       42       42           

Flag	Coverage Δ
pytest	78.05% <ø> (ø)

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