doc: Move distro boot doc to rST

Move this over to the new rST format.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 5e064a4..b3871b1 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -14,6 +14,7 @@
    commands
    config_binding
    devicetree/index
+   distro
    driver-model/index
    global_data
    logging