Explicitly openly-license privacy policy

[cassidyjames]

We've had people asking if they can adapt parts of our privacy policy. This would be more obviously allowed if we just explicitly openly-license it.

[OulipianSummer]

I have created a pull request that adds some new language to the privacy policy page and specifically covers the website's open licensing policy.

There is also a link pointing to the LICENSE.md file in this repo to make it clear where this information can be found. #3729