[WIP] Get started docs
# What does this PR do?

This PR contains the first draft of the Get Started docs as discussed for the new Transformers IA. Some additional thoughts:

- I think adding better signposts on the landing page will help users quickly get to the content they are looking for. For inspiration, maybe we can have cards like [these](https://stripe.com/docs/payments?payments=popular) with links to the pages (cc @mishig25). From the analytics, it looks like people view fine-tuning a pretrained model, fine-tuning for common downstream tasks, installation, and the BERT docs a lot. The first two pages definitely look valuable for getting users up and running with their tasks, but I would love to hear what other pages you think we can add to help guide users down the right path.

- On the Quick tour page, it'd be neat if we can embed a widget there (cc @mishig25) for sentiment analysis so users can play with it directly.
Hey Steven, I believe there was an issue in your merge/rebase and GitHub is having a hard time understanding what happened. Could you close and reopen a new PR without touching at your branch so that we may see the actual differences? Thank you.