Update footer to comply with CNCF/LF trademarks (#1256)

https://www.linuxfoundation.org/trademark-usageSigned-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

layouts/container_footer.html

@@ -2,6 +2,9 @@
 
 <footer>
   <p class="pull-left">
-    &copy; Prometheus Authors 2014-<%= Date.today.year %>
+    &copy; Prometheus Authors 2014-<%= Date.today.year %> | Documentation Distributed under CC-BY-4.0
+  </p>
+  <p class="pull-left">
+     &copy; <%= Date.today.year %> The Linux Foundation. All rights reserved. The Linux Foundation has registered trademarks and uses trademarks. For a list of trademarks of The Linux Foundation, please see our <a href="https://www.linuxfoundation.org/trademark-usage">Trademark Usage</a> page.
   </p>
 </footer>