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

```shell-session
$ source ./venv-build-env.sh
$ cd projects/librobotcontrol/docs
$ doxygen
$ cd ../../..
$ sphinx-build -M html . public
```