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>

Documentation/getting-started.rst

@@ -1,3 +1,4 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
 .. Getting started information is defined in the project README file.
 .. include:: ../README.rst
    :start-after: .. section-begin-getting-started

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
 =========================================