feat(docs): add auto-built API documentation

Add documentation of the API and adopt the existing contents of docs/
into the built documentation.

For the moment we simply *copy* the contents of the README.md file to
docs/index.md and replace the README file with a short stub linking to
the built documentation. Post-merging, additional configuration is
needed in the project's GitLab pages settings to ensure that the
friendly URL is used.

Since the contents of docs/scripts are intended ultimately to be
boilerplate-d and may update outside of this project, this MR excludes
them from strict type checking to avoid introducing spurious errors over
time.

The existing common pipeline functionality for publishing documentation
to GitLab pages is used.

The contents of docs/mkdocs-base.yml and docs/scripts/ are intended to
ultimately form part of any boilerplate and are common to many projects
which use auto-generated documentation. Although it's a bit gnarly to
have them copy-pasted into every library which uses API documentation in
the sunny future world of library boilerplates these will be updated
automatically.