Moving gdt mkdocs to Travis

[mike-potter]

Currently we are doing the automated building of the docs using MkDocs on a remote Phase2 server via Jenkins. It would be better to integrate this directly into this repo via the existing Travis build/test script.

For future reference, here is a blog that has a lot of detail about how to do this: https://mwop.net/blog/2016-01-29-automating-gh-pages.html