Add page up/down bindings to the Markdown preview (#33403)

First time contributor here. 😊

I settled on markdown::MovePageUp and markdown::MovePageDown to match
the names the editor uses for the same functionality.

Closes #30246

Release Notes:

- Support PgUp/PgDown in Markdown previews

crates/markdown_preview/src/markdown_preview.rs

@@ -8,7 +8,13 @@ pub mod markdown_renderer;
 
 actions!(
     markdown,
-    [OpenPreview, OpenPreviewToTheSide, OpenFollowingPreview]
+    [
+        MovePageUp,
+        MovePageDown,
+        OpenPreview,
+        OpenPreviewToTheSide,
+        OpenFollowingPreview
+    ]
 );
 
 pub fn init(cx: &mut App) {