## Summary
We want to have consistency between the Ruff and uv documentation for
the upcoming release. We don't love the Ruff docs, but we'd rather have
consistency and then work towards improving them both, rather than have
two very-different documentation sites that both have weaknesses.
The setup here is simpler than in Ruff as: (1) we don't yet generate any
docs from Rust and (2) we don't try to reuse the README in the uv
documentation (which adds a lot of complexity in Ruff). So the change
here is mostly a 1-to-1 port to MkDocs.
## Test Plan

Whew this is a lot.
The user-facing changes are:
- `uv toolchain` to `uv python` e.g. `uv python find`, `uv python
install`, ...
- `UV_TOOLCHAIN_DIR` to` UV_PYTHON_INSTALL_DIR`
- `<UV_STATE_DIR>/toolchains` to `<UV_STATE_DIR>/python` (with
[automatic
migration](https://github.com/astral-sh/uv/pull/4735/files#r1663029330))
- User-facing messages no longer refer to toolchains, instead using
"Python", "Python versions" or "Python installations"
The internal changes are:
- `uv-toolchain` crate to `uv-python`
- `Toolchain` no longer referenced in type names
- Dropped unused `SystemPython` type (previously replaced)
- Clarified the type names for "managed Python installations"
- (more little things)
Moving the preview features to top-level concepts and pushing the
pip-compatible interface down.
e.g.
<img width="291" alt="Screenshot 2024-06-27 at 7 03 48 AM"
src="500ad97d-899d-4051-b59d-e74786b3a45f">