Add configuration documentation for the build backend and make it the
preview default.
The build backend should generally work with default configuration
unless you want specific features such as flat layout or module
renaming, there is only a dedicated configuration, but no concept or
guide page for the build backend. Once the build backend is stable, we
can update the guide documentation to explain that uv defaults to its
own build backend, but other build backends are also supported.
The uv build backend becomes the default in preview, giving it more
exposure from users and preparing it to make it the default proper. The
current documentation retains warnings that the build backend is in
preview.
To see current uses of `uv_build` on GitHub:
https://github.com/search?q=path%3A**%2Fpyproject.toml+uv_build%3E%3D0&type=code
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Co-authored-by: Zanie Blue <contact@zanie.dev>
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
This adds `poetry-core` as a build backend choice.
## Test Plan
<!-- How was it tested? -->
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Co-authored-by: konstin <konstin@mailbox.org>
Add a preview option `uv init --build-backend uv --preview` that uses
the uv build backend when generating the project. The uv build backend
is in preview, so the option is also guarded by preview and hidden from
the help message and docs.
For https://github.com/astral-sh/uv/issues/3957#issuecomment-2518757563