Revisit licensing

[binkley]

Unclear if moving to CC0 from Public Domain used the correct legal license text and sigils.
At least update the link to the CC org as the CC0 license site may have moved (https://creativecommons.org/public-domain/cc0/).

There are multiple versions of the CC0 license, and the site source is in plain text.
Review version 4.0 at https://github.com/idleberg/Creative-Commons-Markdown/blob/main/4.0/zero.markdown.