[Docs] Optimize Home page format (#3483)

# Optimized home page format for easier reading
## English version: 
### Before:

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/8d077bb7-0179-4e17-970a-87a4877a7b6e)
### After:

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/68b26ff0-4c32-47e7-a105-73c29ceb19c9)

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/88c9bb96-3fa4-4923-8ae4-9db28bf4eb4a)
## Chineses version
### Before:

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/7821cc05-755f-47ef-9ffe-f5d839ee0b6d)
### After:

![image](https://github.com/open-mmlab/mmsegmentation/assets/50650583/f67100cc-6066-4cfa-b4b7-c99a15e4d7a2)