Add support for optional --description in uv init (#10209)

## Summary
Closes #7913 by adding an optional `--description` argument to `uv init`
that fills the description field in the pyproject.toml with the supplied
arg value.

Updated `uv init` docs to describe this new optional argument.
<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan
Added snapshot tests in `uv/crates/uv/tests/it/init.rs` to test this
functionality.
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

docs/reference/cli.md

@@ -589,6 +589,8 @@ uv init [OPTIONS] [PATH]
 <p>While uv configuration can be included in a <code>pyproject.toml</code> file, it is not allowed in this context.</p>
 
 <p>May also be set with the <code>UV_CONFIG_FILE</code> environment variable.</p>
+</dd><dt><code>--description</code> <i>description</i></dt><dd><p>Set the project description</p>
+
 </dd><dt><code>--directory</code> <i>directory</i></dt><dd><p>Change to the given directory prior to running the command.</p>
 
 <p>Relative paths are resolved with the given directory as the base.</p>