Help: Rewrite the cmake-developer.7 find module documentation

As well as the traditional variables, providing imported targets is
suggested, and the relative advantages and disadvantages briefly
discussed.

A mini-tutorial walking through creating a simple find module is
provided.

This changes the recommended version variable from Foo_VERSION_STRING to
Foo_VERSION, because there is really no need to have different variable
names for package version files vs. find modules.  It notes the old
variable name, though, and suggests setting it for compatibility.
Merge topic 'doc-find-module-development' into next

fcdf86d0 Help: Rewrite the cmake-developer.7 find module documentation