🐛 FIX: Add formatting for definition/field/apidoc lists

[chrisjsewell]

closes #158

The CSS for the API doc components is taken from the RTD theme CSS, which I converted to SCSS here: https://gist.github.com/chrisjsewell/0f414e6c13e714f9cf70c36f20efaafa
There can possibly be some "pruning" of unneeded elements?

As noted in the scss, field list should be a different format, but that format is a little tricky (see pydata/pydata-sphinx-theme#193) and not widely used, so I think tackle that in a separate issue/PR

@pradyunsg I also added nitpick mode (to check for missing refs) to the nox sphinx-build commands

[codecov]

Codecov Report

Merging #191 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #191      +/-   ##
==========================================
+ Coverage   91.45%   91.56%   +0.10%     
==========================================
  Files           2        2              
  Lines         234      237       +3     
==========================================
+ Hits          214      217       +3     
  Misses         20       20              

Flag	Coverage Δ
#pytests	91.56% <100.00%> (+0.10%)	⬆️

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Impacted Files	Coverage Δ
sphinx_book_theme/__init__.py	91.51% <ø> (ø)
sphinx_book_theme/launch.py	91.66% <100.00%> (+0.36%)	⬆️

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[chrisjsewell]

Ok merge this and make a release, because I need it for myst-parser