Documentation: Add missing SPDX headers

Two documentation files are missing SPDX headers. Add them, with the
CC-BY-SA-4.0 license that covers all the libcamera documentation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/ipa/ipu3/ipu3-ipa-design-guide.rst

@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
 IPU3 IPA Architecture Design and Overview
 =========================================