Make docstring style consistent

[alexdewar]

PR #45 adds support for auto-building and publishing API documentation using mkdocs.

Currently the docstrings appear to be in a mixture of Google and sphinx styles. While both are supported by the mkdocstrings-python backend (see here), you need to tell mkdocs which style you're using in order for the documentation to render correctly. (To check where there are issues, you can build the docs with mkdocs build and the underlying griffe library will give warnings about malformed docstrings.)

Both styles are fine, so just pick one and run with it. ruff supports enforcing a docstring style, so longer term this would be a good idea.