Update pagekind names in themes.rst: describe story_page, remove generic_page (it's unlikely to actually appear in a site)

Resolves #3691

Author: Kwpolska

docs/theming.rst

@@ -461,9 +461,8 @@ List of page kinds provided by default plugins:
 * list, tags_page
 * post_page
 * page_page
-* story_page
+* story_page (alternate/legacy name for page_page)
 * listing
-* generic_page
 * gallery_front
 * gallery_page