Update of `mkdocs` system and docs content

[davidorme]

This PR was originally to finish #46, by including a couple of simple use case recipes but has ballooned a bit.

1. The mkdocs system had all got behind the most recent versions, so pyproject.toml is updated to the newest versions.

2. The API documentation has been overhauled to sort out:

   • page headers, now only in the markdown stubs, not the module docstrings, which are now all # noqa D415d.
   • the display of individual members and their order. There is something odd here - on my machine the module docstrings were automatically documenting _import_ed components of sdv within each module. For the moment, I've set the members explicitly.
   • An initial safedata system usage page is added, along with the first two use case recipes. This will need expanding and tidying but this closes down the hanging branch for now.

3. The zenodo module has had some return types tweaked to give a cleaner interface.

4. The validator module has had types added for the Filter base class and then has some mypy fixes.

The docs now build cleanly with the most recent mkdocs and some remaining mypy issues are cleared up.

This PR:

• closes #69
• closes #46