chore: CI worfklows for linting docs

[maxrjones]

This PR adds a couple scripts that run in the docs workflow to:

• Check that all top-level functions are included in the mkdocs API documentation
• Check that docstrings use mkdocs rather than sphinx styles admonitions and links

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[maxrjones]

@d-v-b @chuckwondo this adds a fair amount of config/workflow files, so a review on the direction here would be welcome but not necessarily required