Update the release notes

[eric-wieser]

This change:

• Adds documentation for the numba extension support
• Adds a summary of all the changes to multivector inverses
• Adds changelog entries for 1.4.0
• Pulls in two doc changes from updated (and broke some) tools, add some docs.  #389
• Fixes weird indentation in the changelog page
• Updates the sphinx theme for good measure

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This might not be completely exhaustive, but is the things that jumped out to me in 2da66d3...master

https://clifford--403.org.readthedocs.build/en/403/changelog.html

[codecov]

Codecov Report

Merging #403 (e7eb97a) into master (bfc4290) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #403   +/-   ##
=======================================
  Coverage   68.73%   68.73%           
=======================================
  Files          73       73           
  Lines        9319     9319           
  Branches     1202     1202           
=======================================
  Hits         6405     6405           
  Misses       2751     2751           
  Partials      163      163           

Impacted Files	Coverage Δ
clifford/_layout.py	74.94% <ø> (ø)
clifford/_multivector.py	81.18% <ø> (ø)
clifford/numba/__init__.py	100.00% <ø> (ø)
clifford/tools/__init__.py	11.25% <ø> (ø)
clifford/tools/g3c/__init__.py	60.96% <ø> (ø)
clifford/__init__.py	71.25% <100.00%> (ø)

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[hugohadfield]

This looks great Eric