We now have several BT XMLs, many nodes, and questions like #1698 asking about best practices and designing BT nodes / BT trees.

We should add documentation on best practices, available nodes, explanations, and notes for behavior tree designers and behavior tree node designers to help.

I'm also slightly hoping that through this effort to document and explain these topics we may find that there are ways to simplify our nav2_behavior_tree code or take more advantage of smaller behavior tree primitives rather than complex control flow nodes or logic in the factory / navigator.