Add a documentation for what CGAL calls "functor"

[lrineau]

What is called a "functor" in CGAL is not correctly defined in the CGAL documentation, as far as I known. Maybe there should be be a documentation page for it.

https://en.wikipedia.org/wiki/Functor_(disambiguation)

What we call "functor" is an improper naming for a function object. That is fine, if "functor" is defined in the preliminaries of the CGAL manual. AdaptableFunctor is defined, but not "functor".