manuals: add reference to yocto-docs git repository to page footer

To make documentation sources easier to find.

(From yocto-docs rev: 99369e9aca3446a22d710c67f5fd5ace07b713f6)

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

documentation/_templates/footer.html

@@ -5,7 +5,7 @@
         <br> All Rights Reserved. Linux Foundation&reg; and Yocto Project&reg; are registered trademarks of the Linux Foundation.
         <br>Linux&reg; is a registered trademark of Linus Torvalds.
         <br>&copy; Copyright {{ copyright }}
-        <br>Last updated on {{ last_updated }}
+        <br>Last updated on {{ last_updated }} from the <a href="https://git.yoctoproject.org/yocto-docs/">yocto-docs</a> git repository.
     </p>
   </div>
 </footer>