Update docs landing page
This PR updates the docs landing page to better describe what `transformers` is, what it offers, and briefly introduce users to its design. I think this gives a clearer picture of `transformers` and is more impactful than listing all the different tasks supported. Let me know what you think!

There's also a minor issue with the image for custom support. Nils is no longer with us, so we may want to update this image with another member of the team. No big deal though :)
_The documentation is not available anymore as the PR was closed or merged._