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

Added BeagleV Ahead to the front page of docs.

From 115MB to 93.6MB

All images are not optimized, sorted, and stored in a single media folder.

Just playing around.

Newest board family should go first. Microcontroller boards may need separation.