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/vim/src/normal/scroll.rs

@@ -11,13 +11,21 @@ use settings::Settings;
 actions!(
     vim,
     [
+        /// Scrolls up by one line.
         LineUp,
+        /// Scrolls down by one line.
         LineDown,
+        /// Scrolls right by one column.
         ColumnRight,
+        /// Scrolls left by one column.
         ColumnLeft,
+        /// Scrolls up by half a page.
         ScrollUp,
+        /// Scrolls down by half a page.
         ScrollDown,
+        /// Scrolls up by one page.
         PageUp,
+        /// Scrolls down by one page.
         PageDown
     ]
 );