Update sphinx

[nwlandry]

• Up-version sphinx to v6.xxx
• Up-version sphinx-rtd-theme to >= 1.2 to be compatible with Sphinx v6.xxx.
• I added a more organized and concrete contribution guide and eliminated some redundancy in the GH, PyPI, and RTD landing pages. Hopefully this addresses the reviewer's comments in our JOSS submission.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.07 🎉

Comparison is base (5497778) 90.36% compared to head (8519ac8) 90.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #350      +/-   ##
==========================================
+ Coverage   90.36%   90.43%   +0.07%     
==========================================
  Files          41       41              
  Lines        2988     3012      +24     
==========================================
+ Hits         2700     2724      +24     
  Misses        288      288              

see 3 files with indirect coverage changes

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