doc: structure doc/develop/index.rst

Provide sub-chapters for 'Develop U-Boot'

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 0a7e204..b108df8 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -3,13 +3,29 @@
 Develop U-Boot
 ==============
 
+Implementation
+--------------
 
 .. toctree::
-   :maxdepth: 2
+   :maxdepth: 1
 
-   coccinelle
    commands
-   crash_dumps
    global_data
    logging
+
+Debugging
+---------
+
+.. toctree::
+   :maxdepth: 1
+
+   crash_dumps
    trace
+
+Testing
+-------
+
+.. toctree::
+   :maxdepth: 1
+
+   coccinelle