Major documentation theme update and fixes

[omad]

Reason for this pull request

• Major update to the documentation theme
• Upgrade the version of pydata-sphinx-theme
• Amazingly better display on mobile devices/narrow viewports
• Dark mode
• Fixed 180 odd Sphinx Warnings, including rendering issues, missing links, etc.
• Removed the old custom CSS for sidebar nav, since it's now built in.
• Removed the custom HTML mangling code for doing sidebar nav
• Add syntax highlighting for notebook cells used to render the docs
• Renamed the Top Level Nav pages
• Added the Top Level Nav items to the front page of the docs in a grid from sphinx-design

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📚 Documentation preview 📚: https://datacube-core--1676.org.readthedocs.build/en/1676/

[codecov]

Codecov Report

Attention: Patch coverage is 69.38776% with 45 lines in your changes missing coverage. Please review.

Project coverage is 85.64%. Comparing base (62d7cb6) to head (de58dbc).
Report is 16 commits behind head on develop.

Files with missing lines	Patch %	Lines
datacube/api/core.py	79.09%	23 Missing ⚠️
datacube/utils/geometry/gbox.py	16.00%	21 Missing ⚠️
datacube/cfg/api.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1676      +/-   ##
===========================================
- Coverage    85.64%   85.64%   -0.01%     
===========================================
  Files          151      151              
  Lines        15531    15530       -1     
===========================================
- Hits         13301    13300       -1     
  Misses        2230     2230              

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[caitlinadams]

This is great @omad! Thanks for working on it. I've tested the local build and it's working and looks much much better.