Add github pages CI

[macumber]

Add docs folder and port some content over from https://github.com/NREL/OpenStudio-user-documentation/ (https://nrel.github.io/OpenStudio-user-documentation/) (include a few markdown files and images, etc). Add CI task to make docs and post to github pages on merge to master

[tijcolem]

This now has GitHub pages activated for OpenStudioApplication user docs.
https://nrel.github.io/OpenStudioApplication/
A couple of notes on this.

1. branch gh-pages-markdown contain the markdown content.
2. branch gh-pages contain the converted html via mkdocs (v0.11.1).

To enable the automatic markdown build by GitHub, the markdown contents need to be compatible with GitHub Jekyll markdown. For now, the steps to build are to covert the markdown contents gh-pages-markdown to html using mkdocs (v0.11.1) and commit the html to gh-pages branch.