chore: use Canonical's sphinx-docs-starter-pack

[tigarmo]

This commit adds Canonical's starter pack for Sphinx documentation. A consequence of this is that the doc-related dependencies are listed in a requirements file, so we drop those optional-deps from pyproject.toml.

Fixes #187

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[codecov[bot]]

Codecov Report

Merging #197 (54a895b) into main (a88b353) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #197   +/-   ##
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  Coverage   94.73%   94.73%           
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  Files           7        7           
  Lines        1063     1063           
  Branches      194      194           
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  Hits         1007     1007           
  Misses         53       53           
  Partials        3        3           

[tigarmo]

This one isn't use the git submodule?

yeah I figured I'd keep it simple and follow the instructions on the starter-pack site. I hope that next time we "need" to update this it'll be with a proper package hehe