Use Sphinx RTD Theme
- [X] closes #15556 

TOC on the left hand side is pinned during navigation and also expandable/collapsible

<img width="1089" alt="image" src="https://user-images.githubusercontent.com/609873/54065608-3cc81e00-41d8-11e9-830d-124d759bf791.png">


<img width="1099" alt="image" src="https://user-images.githubusercontent.com/609873/54065614-62552780-41d8-11e9-8edc-3bd7db452ca1.png">

