Use `uv` to simplify the local setup instructions in the README

[TimChild]

Update the Setup Locally instructions to use uv instead of venv.

I'm pretty sure you are all using uv now for package management, and it seems like a good idea to recommend to others since it's so convenient to use.

Note: the one liner uv run --with-requirements ... doesn't do hot-reloading (not sure why)... although the hot-reloading wasn't working well for me even with a full venv. I included instructions for methods of setup anyway.