Made documentation more accurate and useful, including a Doxygen config.

[b-adkins]

I have been using BehaviorTree.CPP and .ROS2 and have been finding small issues with the documentation.

Changes

Fixed documentation to be more accurate and to include information that I had to read the source to figure out. Added Doxygen config and fixed broken Doxygen tags.

• "A Groot2Publisher is included in TreeExecutionServer." Because there is no mention of it, PImpl hides its existence from the header file, and it makes no log messages, I had no idea it existed and wasted a ton of time adding my own.
• bt_executor_parameters.md. As I made my own, I realized that the example YAML wasn't quite the right types. (I'm not using plugins, I have no idea what the valid type is there; it kept erroring out at my attempts.)
• Listed the "basic ports" referred to in BT::RosActionNode.

Doxygen Config

• Includes the Markdown files properly, with the README as the front page and functioning links
• Respects abstraction (following the underlying philosophy of the PImpl pattern used in this library):
  • Source files are only included from the samples repo
  • ROS2 interface files (.action, .msg) are conveniently available in the file browser