Update Python docs to use `uv` instead of `poetry`

This PR updates the Python guides to use uv for dependency management.

The "Setting up a Python Environment" guide is mostly the same. Because uv uses venv by default, fewer configuration steps are required.
The "Multi-stage Builds" guide is a more involved change.
The poetry_new partial is replaced with a new uv_init partial, and all of its references are updated.
The ["Getting Started"] guide for Django still uses pip + venv, and is updated only to mention uv as an alternative (as it currently does for Poetry)
The remaining changes are simple replacements of poetry add → uv add, which does the same thing.

superfly / docs