Stabilize the uv build backend (#14311)
The uv build backend has gone through some feedback cycles, we expect no
more major configuration changes, and we're ready to take the next step:
The uv build backend in stable.
This PR stabilizes:
* Using `uv_build` as build backend
* The documentation of the uv build backend
* The direct build fast path, where uv doesn't use PEP 517 if you're
using `uv_build` in a compatible version.
* `uv build --list`, which is limited to `uv_build`.
It does not:
* Make `uv_build` the default on `uv init`
* Make `--package` the default on `uv init`
