Generate environment variables doc from code (#8493)

## Summary

Resolves #8417

I've just begun learning procedural macros, so this PR is more of a
proof of concept. It's still a work in progress, and I welcome any
assistance or feedback.

crates/uv-dev/src/generate_options_reference.rs

@@ -34,7 +34,6 @@ struct CombinedOptions {
 
 #[derive(clap::Args)]
 pub(crate) struct Args {
-    /// Write the generated output to stdout (rather than to `settings.md`).
     #[arg(long, default_value_t, value_enum)]
     pub(crate) mode: Mode,
 }