ENH: Add Creative Commons license

[westurner]

https://creativecommons.org/choose/

• [x] Get HTML code

<a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-sa/4.0/88x31.png" /></a><br /><span xmlns:dct="http://purl.org/dc/terms/" property="dct:title">WRD R&D Documentation</span> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">Creative Commons Attribution-ShareAlike 4.0 International License</a>.

• [x] Download/cache 88x31.png for offline support (cc-by-sa-4.0-88x31.png)
• [x] _templates/layout.html: {% extends '!layout.html' %}
• [x] _templates/layout.html: Add CC HTML
• [x] Add copyright.rst