docs: env var doc improvements (#8777)

## Summary

* Env docs now support anchors, which allows sending a link to someone
with a direct reference to an env var or cross-reference them in the
docs.
* Marked additional env vars as hidden from the docs due to their
internal use
* Updates some tests still using literals to use the static env vars

## Test Plan

<img width="1370" alt="env_var_anchors"
src="https://github.com/user-attachments/assets/52ae1caa-5199-4798-9eb5-81b8f5b57c24">

crates/uv-dev/src/generate_env_vars_reference.rs

@@ -88,7 +88,9 @@ fn generate() -> String {
             })
             .collect::<Vec<_>>()
             .join("\n");
-        output.push_str(&format!("- `{var}`: {doc}\n"));
+        output.push_str(&format!(
+            "- <a id=\"{var}\"></a> [`{var}`](#{var}): {doc}\n"
+        ));
     }
 
     output