Update docs/Makefile to use uv (and push upstream)

[benhoyt]

For when someone is bored:

Currently the main Makefile uses uv, which is fast. However, docs/Makefile, which comes from the Canonical starter pack, uses regular venvs with pip install, which is slow. It'd be great to fix the docs one to use uv as well, and then propose changes back upstream to the Canonical starter pack repo. We probably want to keep ours in sync, so probably wait till upstream is merged before merging our PR.