Redesign dotty.epfl.ch/index.html

[TheElectronWill]

After #7595, the docs got a new theme (implemented in #7612).
The front page still needs to be changed to something more attractive (and that fits the colors and fonts of the new docs).

I think it could contain (not necessarily in that order):

• the logo and label (of course)
• links to github, docs, etc.
• a quick overview of some new features. IIRC Martin Odersky did a poll on Twitter some time ago to elect the "top" dotty features, and also gave his own opinion. We could compare Scala 2 code with Dotty code to emphasize the improvement.
• a list of some (other) features
• how to install dotty
• how to start using it (sbt, scastie)

[abgruszecki]

@TheElectronWill Thanks for your work! In the interest of keeping our issue queue clean, I'm going to close this issue. If you want to discuss a related topic, feel free to open another issue or just a PR!