docs: add edge channel for document-driven mode

Author: atinux

docs/content/3.guide/1.writing/7.document-driven.md

@@ -9,6 +9,10 @@ This mode creates a direct binding between the `content/` directory and pages.
 
 It also offers `page`, `navigation`, `surround` and `globals` variables.
 
+::alert{type="info"}
+This feature is currently available on the [edge channel](/guide/migration/edge-channel).
+::
+
 ## Enable the mode
 
 ### Existing project