Licensing pass to make all annotations clear across the bodies of work

[hollasch]

Quoting from Vijay Vaidyanathan:

The repository carries a CC0 1.0 Universal dedication in `COPYING.txt`,
the CHANGELOG records the move to CC0, and every C++ source file carries
a per-file public domain dedication in its header. But the three book
HTML files still carry the line "Copyright 2018-2024 Peter Shirley. All
rights reserved." in their title blocks. I read the first as governing
and the second as a line that predates the relicense and never got
updated - but I would much rather ask than assume, since a derivative
edition is exactly the case where the difference matters.

The fix here is probably to update the copyright line heading each book's Markdeep source.