docs/sphinx: License content with CC-BY-4.0

Creative Commons is a more common license than GPL for documentation purposes.
Switch to using CC-BY-4.0 to explicitly permit re-purposing and remixing of
the content.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Lars Kurth <lars.kurth@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

docs/index.rst

@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: CC-BY-4.0
+
 The Xen Hypervisor documentation
 ================================