common_settings: migrate document menu to menu_order ordering

This is necessary in order to allow the language support module to be
added to the menu outside of the common_settings menu table definition.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

frontend/ui/elements/reader_menu_order.lua

@@ -72,6 +72,11 @@ local order = {
         "----------------------------",
         "status_bar",
     },
+    document = {
+        "document_auto_save",
+        "document_save",
+        "document_end_action",
+    },
     device = {
         "keyboard_layout",
         "time",