feat: draft implementation of extension and theme

[tigarmo]

This commit continues the bootstraping by adding a bunch of things:

• Add theme assets (from the starter pack);
• Add extension code (from the prototype);
• Configure the project build to bundle the theme assets;
• Update docs/conf.py to use the extension. The goal there is to be able to correctly style/configure the docs with minimal information (the docs themselves are still the placeholder from Starbase).

This is a work-in-progress; not all options supported by the starter pack are supported here yet.

• [ ] Have you followed the guidelines for contributing?
• [ ] Have you signed the CLA?
• [ ] Have you successfully run tox?

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[tigarmo]

@ru-fu I'm going to merge this as we're near the end of the pulse - I'm going to create some follow-up issues but feel free to review and comment anyway if you want, we can still improve things!

[ru-fu]

FYI @dviererbe (if you haven't seen yet).