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/outline_panel/src/outline_panel.rs

@@ -65,17 +65,28 @@ use worktree::{Entry, ProjectEntryId, WorktreeId};
 actions!(
     outline_panel,
     [
+        /// Collapses all entries in the outline tree.
         CollapseAllEntries,
+        /// Collapses the currently selected entry.
         CollapseSelectedEntry,
+        /// Expands all entries in the outline tree.
         ExpandAllEntries,
+        /// Expands the currently selected entry.
         ExpandSelectedEntry,
+        /// Folds the selected directory.
         FoldDirectory,
+        /// Opens the selected entry in the editor.
         OpenSelectedEntry,
+        /// Reveals the selected item in the system file manager.
         RevealInFileManager,
+        /// Selects the parent of the current entry.
         SelectParent,
+        /// Toggles the pin status of the active editor.
         ToggleActiveEditorPin,
-        ToggleFocus,
+        /// Unfolds the selected directory.
         UnfoldDirectory,
+        /// Toggles focus on the outline panel.
+        ToggleFocus,
     ]
 );