Change the documentation theme to Furo

[jstasiak]

The default one feels a bit outdated to me, let's try this[1].

I initially wanted to go with the RTD theme but it doesn't support dark mode natively[2].

The theme is used in Python Developer's Guide[3] and it looks nice in my opinion.

[1] https://github.com/pradyunsg/furo#elevator-pitch
[2] readthedocs/sphinx_rtd_theme#224
[3] https://devguide.python.org/

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.22%. Comparing base (78bb621) to head (ad625d5).
Report is 7 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #244   +/-   ##
=======================================
  Coverage   95.22%   95.22%           
=======================================
  Files           1        1           
  Lines         565      565           
  Branches       96       96           
=======================================
  Hits          538      538           
  Misses         20       20           
  Partials        7        7           

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[jstasiak]

Old: https://injector--243.org.readthedocs.build/en/243/
New: https://injector--244.org.readthedocs.build/en/244/

[jstasiak]

One blocker (the localtoc.html issue): https://injector--244.org.readthedocs.build/en/244/changelog.html

Leaving this for later