Add automatic API reference (GSI-1892)

[Cito]

This PR adds an automatically generated API reference to the existing documentation.

I tried using mkdocs-autoapi first, but this looked ugly. Also, pydantic fields did not show up. In principle, griffe_pydantic should do this, but it did not work properly for me.

Therefore, I decided to use pdoc instead (as a separate build step from mkdocs) which looks much nicer.

The documentation can be built with CI or with tox -e docs. This will first generate the API reference under /api which makes it possible to link to it from the narrative documentation without target warnings when running mkdocs build. Afterwards, the API reference is moved from the docs directory to the build (site) directory to have a functional website that is deployed via GitHub pages.

The PR also fixes some docstring issues that showed up as warnings when running mkdocs-autoapi.

[coveralls]

Pull Request Test Coverage Report for Build 22232393439

Details

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.932%

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Totals
Change from base Build 22110184979:	0.0%
Covered Lines:	3392
Relevant Lines:	3650

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💛 - Coveralls

[Cito]

Note: To test this, run tox -e docs and python -m http.server and browse to the site directory.