feat(docs): add auto-built API documentation

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

The webapp-boilerplate has existing support for auto-API docs and this MR ports the approach used there.

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.

In any given MR, the—poorly named—"View app" or "View latest app" button in the MR UI can be used to preview the new API docs:

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.

Closes #49 (closed)