docs: use the right ops-scenario for building the docs

[tonyandrewmeyer]

This PR fixes the version of ops-scenario used when building the docs.

Previously, the published version of ops-scenario was used for generating the documentation, and the generated requirements file was used for local builds but not for read-the-docs.

That breaks if there are local incompatible changes (such as renaming a module), and is generally wrong anyway.

With this PR, the local version is loaded, and read-the-docs uses the same requirements file as building locally. A few minor type annotations were adjusted to remove remaining warnings (I don't know why Sphinx couldn't figure out where Path was coming from, but keeping it in the pathlib namespace is generally nicer anyway).

Split out from #1467.