JsonExporter docs out of date

[tony-p]

In the docs about groot, it gives instructions how to create the json exports to visualize custom types https://www.behaviortree.dev/docs/tutorial-basics/tutorial_11_groot2

This has changed a few times since, and reading the current source code docs https://github.com/BehaviorTree/BehaviorTree.CPP/blob/22929a25473fee1e852fa06734e947e889b433b4/include/behaviortree_cpp/json_export.h#L13 , the instructions are closer to the <=4.3.5 instructions (with an extra macro and wrapper function) than the current >4.3.5 instructions in the docs.

This leads to a lot of confusion about the right way to implement these.