doc: Move driver model docs under develop/

These docs are useful for developers, not users. Move them under that
section.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 84914bb..444df67 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -10,6 +10,7 @@
    :maxdepth: 1
 
    commands
+   driver-model/index
    global_data
    logging
    menus