Sync license notes.

[davidlehn]

License info across the site needs to get in sync:

• https://github.com/json-ld/json-ld.org/blob/master/LICENSE.md refers to https://www.w3.org/Consortium/Legal/copyright-software.
• https://github.com/json-ld/json-ld.org/blob/master/CONTRIBUTING.md refers to http://www.w3.org/Consortium/Legal/copyright-documents
• Mostly all pages refer to https://creativecommons.org/about/cc0

(I also still wonder about the LICENSE.md doc saying "all" when some 3rd party software and libs are used with their own licenses.)

[gkellogg]

License should be active for all sub-directories that don't have their own license. We should probably add a LICENSE.md file in /playground to cover that particular code, and perhaps make it clear that license holds for all directories not having a more specific license.

CONTRIBUTING should definitely be synchronized, and HTML pages should refer to the appropriate license file.

[davidlehn]

3rd party stuff is also in at least /static/ and /fonts/. I don't think this needs to get too complex. Just need to note that a handful of files are under their own license.