add documentation for `mapping_sort`

docs/files/templates.md

@@ -210,7 +210,7 @@ In addition, templates also have a few special attributes that they can use:
 
     <div id="move-prefix" />The `move_prefix` attribute can be used to specify a list or comma-separated string of 
     prefixes to move to the end of the collection/playlist name for sorting. This changes the template variables
-    `collection_sort` and `playlist_sort`.
+    `collection_sort`, `playlist_sort`, and `mapping_sort`.
 
     ???+ example "Example"
 
@@ -241,6 +241,7 @@ Every template also has access to these template variables:
 * Either `<<collection_name>>`, `<<playlist_name>>`, or `<<overlay_name>>` which is the name of the definition.
 * `<<mapping_name>>` is the original mapping name for the definition in the YAML file.
 * Either `<<collection_sort>>` or `<<playlist_sort>>` which is the name of the definition after `move_prefix` is applied.
+* `<<mapping_sort>>` which is the original mapping name for the definition after `move_prefix` is applied.
 * `<<library_type>>` which is the library type (`movie`, `show`, `artist`, `video`).
 * `<<library_name>>` which is the name of the library.
 * All template variables can append `_encoded` to the variable name to use a URL encode version of the variable. ex.