manuals: mention license information in footer

Previously, the license for the manuals was only
stated in the SPDX headers at the beginning of source files

(From yocto-docs rev: cef60f23d5601a18d7ba7e894af2aa459fb2efb8)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/conf.py

@@ -28,7 +28,7 @@ release = current_version
 
 # -- Project information -----------------------------------------------------
 project = 'The Yocto Project \xae'
-copyright = '2010-%s, The Linux Foundation' % datetime.datetime.now().year
+copyright = '2010-%s, The Linux Foundation, CC-BY-SA-2.0-UK license' % datetime.datetime.now().year
 author = 'The Linux Foundation'
 
 # -- General configuration ---------------------------------------------------