doc: Add mention of the /config binding

The devicetree binding files are in their own directory and use a simple
text format. Add a link for the binding for the /config node, since it
is otherwise hard to find.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 2a32645..24056b1 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -12,6 +12,7 @@
    bloblist
    ci_testing
    commands
+   config_binding
    devicetree/index
    driver-model/index
    global_data