[Doc] Fix doc index and update chinese doc accordingly (#775)

* Update index.rst

* Create s3dis_sem_seg.md

* Create scannet_sem_seg.md

* Update index.rst

docs/datasets/index.rst

 .. toctree::
    :maxdepth: 2
 
-   scannet_det.md
    waymo_det.md
+   scannet_det.md
+   scannet_sem_seg.md
+   s3dis_sem_seg.md

docs_zh-CN/datasets/index.rst

 .. toctree::
    :maxdepth: 2
 
-   scannet_det.md
    waymo_det.md
+   scannet_det.md
+   scannet_sem_seg.md
+   s3dis_sem_seg.md

docs_zh-CN/datasets/s3dis_sem_seg.md

+# 3D 语义分割 S3DIS 数据集

docs_zh-CN/datasets/scannet_sem_seg.md

+# 3D 语义分割 ScanNet 数据集