Add docs contents at README.md to easily find documents.
Issue: #2664 

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/0763e108-f095-44d1-8e2a-ba2e83f02625)

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/088e0945-55b4-4d3f-97bb-02e39dfafb5e)

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/109e0320-d6a9-405f-b169-87acb5a0f94d)

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/d1d72bde-40c0-451a-be6e-010f1f2d193b)