Sphinx API docs improvements

[Kami]

This pull request includes the following sphinx API docs improvements:

• Move api docs generation from tox file to sphinix config. This seems to be the preferred method. It allows us to re-use the same tox target locally and on CI and it also simplifies RTD approach.
• Update sphinx-apidocs options to generate separate file for each modules. This makes linking to docs for a specific module easier.
• Update sphinx-apidocs options to exclude test modules from the api docs.

Those changes should also drastically speed up the docs build time on ReadTheDocs (down to ~200 seconds from ~700 seconds).

[codecov-commenter]

Codecov Report

Merging #1937 (d7f0a8b) into trunk (794818c) will not change coverage.
The diff coverage is n/a.

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##            trunk    #1937   +/-   ##
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  Coverage   83.18%   83.18%           
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  Files         352      352           
  Lines       81329    81329           
  Branches     8579     8579           
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  Hits        67652    67652           
  Misses      10873    10873           
  Partials     2804     2804