[Docs] Improve docs style

[Junjun2016]

Modifications

1. Use adapted pytorch_sphinx_theme
2. Fix versions of some docs dependencies
3. Use myst_parser to parse markdown files instead of recommonmark
4. Apply new notes and warning directives to docs

[codecov]

Codecov Report

Merging #879 (1092314) into master (cae715a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #879   +/-   ##
=======================================
  Coverage   89.05%   89.05%           
=======================================
  Files         111      111           
  Lines        6051     6051           
  Branches      969      969           
=======================================
  Hits         5389     5389           
  Misses        467      467           
  Partials      195      195           

Flag	Coverage Δ
unittests	89.05% <ø> (ø)

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