Website and docs updates

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

website/tutorials.json

@@ -12,9 +12,18 @@
      {
       "id": "render_textured_meshes",
       "title": "Render Textured Meshes"
+     },{
+      "id": "fit_textured_mesh",
+      "title": "Fit a mesh with texture via rendering"
      }, {
       "id": "camera_position_optimization_with_differentiable_rendering",
-      "title": "Camera Position Optimization"         
+      "title": "Camera Position Optimization"
      }
-  ]
-}
+  ],
+  "Dataloaders": [
+      {
+         "id": "dataloaders_ShapeNetCore_R2N2",
+         "title": "Data loaders for ShapeNetCore and R2N2"
+      }
+   ]
+}