Previously, new files in test directories named `Build*` on
case-insensitive filesystems were being incorrectly ignored.
While at it, add more comments justifying some entries.
Provide readers following the tutorial on `cmake.org` a direct link to
download the tutorial's source examples. Previously readers had to
fetch the CMake source tree separately and look for the tutorial source
examples inside it.
Fixes: #22475
Brad King