gpui: Store action documentation (#33809)

Closes #ISSUE

Adds a new `documentation` method to actions, that is extracted from doc
comments when using the `actions!` or derive macros.

Additionally, this PR adds doc comments to as many action definitions in
Zed as possible.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/feedback/src/feedback.rs

@@ -11,9 +11,13 @@ pub mod system_specs;
 actions!(
     zed,
     [
+        /// Copies system specifications to the clipboard for bug reports.
         CopySystemSpecsIntoClipboard,
+        /// Opens email client to send feedback to Zed support.
         EmailZed,
+        /// Opens the Zed repository on GitHub.
         OpenZedRepo,
+        /// Opens the feature request form.
         RequestFeature,
     ]
 );