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/markdown_preview/src/markdown_preview.rs

@@ -9,10 +9,15 @@ pub mod markdown_renderer;
 actions!(
     markdown,
     [
+        /// Scrolls up by one page in the markdown preview.
         MovePageUp,
+        /// Scrolls down by one page in the markdown preview.
         MovePageDown,
+        /// Opens a markdown preview for the current file.
         OpenPreview,
+        /// Opens a markdown preview in a split pane.
         OpenPreviewToTheSide,
+        /// Opens a following markdown preview that syncs with the editor.
         OpenFollowingPreview
     ]
 );