Setup automated testing across multiple Sphinx and docutils versions

[pradyunsg]

The core idea here is, hopefully, obvious: This codebase needs to be tested to ensure that the output of builds using this theme is "correct".

The pieces that are needed for this would be:

• [ ] The matrix-computation setup for determining the relevant Sphinx and docutils versions to test against.
• [ ] The actual tests
  • [ ] Validating the output changes of the different knobs provided.
  • [ ] Validating the functional behaviours of each of the functional features provides.
  • [ ] Validating the bare-minimums of accessibility[^1].
  • [ ] Validating the visual presentation of specific elements.

[^1]: ... while being cognizant of measurability bias and that this alone doesn't mean that the answer for "is this accessible" is "yes" .