Add new doc and new tutorials to website

Summary: Recent additions need to be included.

Reviewed By: nikhilaravi

Differential Revision: D30223717

fbshipit-source-id: 4b29a4132ea6fb7c1a530aac5d1e36aa61c663bb

website/sidebars.json

 {
   "docs": {
     "Introduction": ["why_pytorch3d"],
-    "Data": ["meshes_io", "datasets", "batching"],
+    "Data": ["io", "meshes_io", "datasets", "batching"],
     "Ops": ["cubify"],
     "Visualization": ["visualization"],
     "Renderer": ["renderer", "renderer_getting_started", "cameras"]

website/tutorials.json

@@ -24,6 +24,12 @@
       }, {
        "id": "camera_position_optimization_with_differentiable_rendering",
        "title": "Camera Position Optimization with Differentiable Rendering"
+       },{
+       "id": "fit_textured_volume",
+       "title": "Fit a volume via raymarching"
+       },{
+       "id": "fit_simple_neural_radiance_field",
+       "title": "Fit a simplified NeRF via raymarching"
       }
    ],
    "Dataloaders": [