Fix documentation (pydata-sphinx-theme broke our build) + migrate to myst-nb

[ma-sadeghi]

• Switched over to myst-nb from nbsphinx as it seems to be more robust and has more useful features
• Pinned pydata-sphinx-theme version to 0.9 to it won't break again!
• Added a table of contents to the sidebar of the examples page
• Fixed night mode
• Removed "getting started", moved "installation" and "basic usage" up to root level for visibility
• Lots of other aesthetic enhancements, similar to what we recently did to OpenPNM docs

[codecov]

Codecov Report

Merging #731 (5fa0606) into dev (3b5f333) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##             dev    #731   +/-   ##
=====================================
  Coverage   83.8%   83.8%           
=====================================
  Files         36      36           
  Lines       4035    4035           
=====================================
  Hits        3385    3385           
  Misses       650     650