doc: Move devicetree control doc to rST

Move this to rST format, largely unchanged to start with. Add an index
for this topic, as well as an empty intro.

Note this patch does not include updates! Is it just a conversion to the
new format. See the next patch.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchart <xypron.glpk@gmx.de>
diff --git a/doc/develop/devicetree/index.rst b/doc/develop/devicetree/index.rst
new file mode 100644
index 0000000..fa5db3e
--- /dev/null
+++ b/doc/develop/devicetree/index.rst
@@ -0,0 +1,13 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Devicetree in U-Boot
+====================
+
+The following holds information on how U-Boot makes use of devicetree for
+build-time and runtime configuration.
+
+.. toctree::
+   :maxdepth: 2
+
+   intro
+   control