toc: Document permalink_prepend

Author: ferdnyc

docs/extensions/toc.md

@@ -174,6 +174,10 @@ The following options are provided to configure the output:
 * **`permalink_title`**:
     Title attribute of the permanent link. Defaults to `Permanent link`.
 
+* **`permalink_prepend`**:
+    Set to `True` if the generated permalinks should be placed at the
+    start of the header, rather than at the end. Default is `False`.
+
 * **`baselevel`**:
     Base level for headers. Defaults to `1`.