Reduce librobotcontrol for now and cleanup each board intro

See merge request docs/docs.beagleboard.io!69

Disabling `exhale` because it is just overloading things right now.

Hidden in this commit are a couple of other items:
* Enable .md documents for CONTRIBUTING.md to avoid same content not on
  docs.beagleboard.org
* Added graphviz to the venv setup, but that doesn't do much as I think
  it was already there