* add support for both command line arg and env var to set NGRAPH_WARINGS_AS_ERRORS'

* Add mention of blob ref of original file from caffe2

* Mention location of source listing originally from LLVM project

Reused from Caffe 2 (Apache Licensed)

[Link](https://github.com/caffe2/caffe2/blob/master/cmake/Modules/FindCuDNN.cmake)

- Adds Sphinx docs support.

- Sphinx and Doxygen docs now built by CI.

- Sphinx and Doxygen docs now installed by `make install`.

add cmake finder for graphviz dot program and then conditionally compile visualization based on finding