Switch to uv in Dockerfile and workflows

[otargowski]

Replacing pip with uv pip in the Dockerfile provided a 155s -> 8s speedup for the requirements.txt installation step on my modern cpu. On an i7 2640M from 2011 it now takes 15s. The only proper issue was the lack of --user support, which was handled by using a proper venv.

I also cleaned up the workflows somewhat with a composite action for setting up the environment and updated the requirements for generating docs (otherwise they wouldn't work, perhaps due to a version conflict).

I chose not to replace pip with uv pip in e.g. the pre-commit setup instructions in the README.

All workflows have been on my fork and they seem to work as well as they should without e.g. adequate API tokens to Transifex.