Add keywords to the Default editor shortcuts page

This improves search results when trying to look for the list of
editor shortcuts.

(cherry picked from commit e5b8585130dc5dedd43a88d4d237f25386fd2da1)

getting_started/editor/default_key_mapping.rst

@@ -1,3 +1,6 @@
+.. meta::
+    :keywords: cheatsheet, cheat sheet, shortcut
+
 .. _doc_default_key_mapping:
 
 Default editor shortcuts