docs: migrate from poetry to uv
Our docs are built with MkDocs, which requires Python and several deps.
Previously those deps were managed with Poetry, which is also written in Python.
This commit replaces Poetry with `uv`, a Rust-based Python
project/package manager, and thus removes several steps from the docs
build process.
Before:
<install Python>
<install pipx>
pipx install poetry
poetry install
poetry run -- mkdocs serve
After:
<install uv>
uv run mkdocs serve