feat: add some basic documentation

The common CI pipeline has support for mkdocs-based documentation and so
add a basic generated mkdocs config. Base the mkdocs config on the
config which is likely to land in the copier template to minimise future
disruption.

We need to override some parts of the standard jobs to "inject" the
documentation generated by the OpenAPI templates into them. We also need
to add "fake" index.html files which take the place of the real
documentation which will be injected in.