Summary:
- Added sbranson's fit mesh tutorial to the website
- Updated rendering docs with info about texturing and new shader types.

TODO:
- add pointcloud rendering tutorial to the website as well (https://github.com/facebookresearch/pytorch3d/blob/master/docs/tutorials/render_colored_points.ipynb)
- docs for camera
- update some tutorials which depended on the Textures from structures.

Reviewed By: gkioxari

Differential Revision: D23143977

fbshipit-source-id: 6843c9bf3ce11115c459c64da5b0ad778dc92177