Use uv sync & uv run in docs build

[browniebroke]

Description

The offical RTD guide was written a few months ago and I realised after merging https://github.com/cookiecutter/cookiecutter-django/pull/5438 that we can in fact simplify a bit more.

Also upstreamed these changes to RTD: https://github.com/readthedocs/readthedocs.org/pull/11655

Checklist:

• [x] I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• [x] I've updated the documentation or confirm that my change doesn't require any updates

Rationale

Simpler build steps

[browniebroke]

I would expect the --no-dev and --frozen parameters here

Good point! Fixed

[foarsitter]

Forgot that we don't have dev dependencies xD

Will add some soon ;)