Docs have not been published for years - Make sure the docs are built after every merge

[DanielNoord]

I just found out that our documentation hasn't been deployed since 2022:
https://github.com/PyCQA/isort/deployments

This is because it uses a quite complex setup of portray and Github Pages that is not documented anywhere.

@staticdev Shall we just migrate this to ReadTheDocs? It is much simpler and also ensures documentation gets built on every PR.

[staticdev]

@DanielNoord about that, I have been using successfully mdbook on my new projects to build documentation. It is written in Rust so it is quite fast, I recommend taking a look. There is also https://github.com/peaceiris/actions-mdbook that can automated the build in the ci/cd.

[DanielNoord]

@staticdev It seems kind of hard to integratie that into the development environment as you'd also need to have cargo or mdbook available, which you can't do via uv.

portray apparently also doesn't work on the version we support so basically we currently can't build the docs...

Migrating to sphinx also seems like a significant effort unfortunately.