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>

doc/develop/driver-model/index.rst

@@ -3,6 +3,10 @@
 Driver Model
 ============
 
+The following holds information on the U-Boot device driver framework:
+driver-model, including the design details of itself and several driver
+subsystems
+
 .. toctree::
    :maxdepth: 2
 

doc/develop/index.rst

@@ -10,6 +10,7 @@ Implementation
    :maxdepth: 1
 
    commands
+   driver-model/index
    global_data
    logging
    menus

doc/index.rst

@@ -38,17 +38,6 @@ want to contribute to U-Boot.
 
    develop/index
 
-Driver-Model documentation
---------------------------
-
-The following holds information on the U-Boot device driver framework:
-driver-model, including the design details of itself and several driver
-subsystems.
-
-.. toctree::
-   :maxdepth: 2
-
-   driver-model/index
 
 U-Boot API documentation
 ------------------------