Add documentation for rule templates

[rkoshak]

The development of rule templates is largely missing from the docs and there have been some big improvements to authoring templates added recently in core with support for file based templates and multiple rule templates in one file/posting on MainUI.

But I'm not sure the best place to put it. Does this belong under Developer Guide? A new page under Configuration?

What they are is already covered in concepts, getting started, etc. but how to create one doesn't exist outside of the forum.