Add more subsections to main doc
# What does this PR do?

This PR adds a subsection right before the list of supported models & the big table of supported frameworks for each model. Merging this PR would change the "welcome" doc page as follows:

![pic1](https://user-images.githubusercontent.com/23423619/118638456-c4edd900-b7ce-11eb-8191-aa9bfc62248a.png)

and 

![pic2](https://user-images.githubusercontent.com/23423619/118638477-ca4b2380-b7ce-11eb-8dbe-f9d68b3c6908.png)

The motivation for this PR is mainly to be able to better link to all supported models and framework. *E.g.* when asking which models are supported by Flax, it's nice to have a direct link instead of having to scroll down 