x/website: add a link to Creative Commons Attribution 3.0 License

[perillo]

Currently, the license for documents in content/static/godoc.html and internal/dl/tmpl.go is in plain text. I think it should be a link to the license definition, e.g.
https://creativecommons.org/licenses/by/3.0/.

[agnivade]

/cc @dmitshur @andybons

[dmitshur]

It's linked in the README and at https://developers.google.com/terms/site-policies#restrictions, so it seems reasonable to link in the footer too.

[bcmills]

CC @cnoellekb

[rhnvrm]

I would like to take up this issue.

[dmitshur]

@rhnvrm Feel free to send a change to the x/website repo. If it's your first time contributing, read over the Contribution Guide for information.

[gopherbot]

Change https://golang.org/cl/163746 mentions this issue: Add missing links for creative commons license

[rhnvrm]

@bradfitz @agnivade, thank you for reviewing!