add example(s) showing Albrecht's innovative overlap resizable technique to the "How does resizing work?" documentation and tests See "resizable question" original discussion thread under: https://www.fltk.org/newsgroups.php?gfltk.general+v:39635
... as discussed in fltk.coredev for easier maintenance. Also edit some minor issues.
add resize chapter to documentation, plus images, based on Article #415: How does resizing work? https://www.fltk.org/articles.php?L415 see also https://www.fltk.org/str.php?L3433
