- Removes "experimental" labels from command documentation
- Removes preview warnings
- Removes `PreviewMode` from most structs and methods — we could keep it
around but I figure we can propagate it again easily where needed in the
future
- Enables preview behavior by default everywhere, e.g., `uv venv` will
download Python versions

This PR moves us to the Linux strategy for our global directories on
macOS. We both feel on the team _and_ have received feedback (in Issues
and Polls) that the `Application Support` directories are more intended
for GUIs, and CLI tools are correct to respect the XDG variables and use
the same directory paths on Linux and macOS.

Namely, we now use:

- `/Users/crmarsh/.local/share/uv/tools` (for tools)
- `/Users/crmarsh/.local/share/uv/python` (for Pythons)
- `/Users/crmarsh/.cache/uv` (for the cache)

The strategy is such that if the `/Users/crmarsh/Library/Application
Support/uv` already exists, we keep using it -- same goes for
`/Users/crmarsh/Library/Caches/uv`, so **it's entirely backwards
compatible**.

If you want to force a migration to the new schema, you can run:

- `uv cache clean`
- `uv tool uninstall --all`
- `uv python uninstall --all`

Which will clean up the macOS-specific directories, paving the way for
the above paths. In other words, once you run those commands, subsequent
`uv` operations will automatically use the `~/.cache` and `~/.local`
variants.

Closes #4411.

---------

Co-authored-by: Zanie Blue <[email protected]>

These are global and non-specific to the `pip` API, so I think they
should be elevated.

- Ran `UV_CONCURRENT_DOWNLOADS=1 cargo run pip list`; verified that
`downloads` resolved to 1.
- Added `concurrent-downloads = 5` under `[tool.uv]` in
`pyproject.toml`; ran `cargo run pip list`; verified that `downloads`
resolved to 5.
- Ran `UV_CONCURRENT_DOWNLOADS=1 cargo run pip list`; verified that
`downloads` resolved to 1.

Executable name requests were being treated as explicit requests to
install into system environments, but I don't think it should be as it's
implicit what environment you'll end up in. Following #4308, we allow
multiple executables to be found so we can filter here.

Concretely, this means `--system` is required to install into a system
environment discovered with e.g. `--python=python`. The flag is still
not required for cases where we're not mutating environment.

This is a fallback mode that we supported when we decided to use PEP 517
builds by default. I can't find a single reference to it on GitHub or in
our issue tracker, so I want to drop support for it as part of v0.3.0.

First, I synced the documents manually following the output of `diff`
then replaced the old one with a link.

🚀
[Rendered](https://github.com/astral-sh/uv/tree/zb/3-readme?tab=readme-ov-file#uv)