improve Copyright and CC

The inc/footer.default.html now has a common Copyright and CC BY-SA
4.0 License section for all generated HTML files.

All markdown files have a similar Copyright and CC
at the bottom in the form of an HTML comment.

2020/ferguson1/HACKING.md

@@ -519,3 +519,18 @@ done for maximum number of movements before game over. These would both be
 rather easy to implement but personally I would only consider the max number of
 evasions (I just don't have many more bytes left!) - unless maybe that there is
 no default limit on max moves.
+
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