Website contains outdated content that should be removed

[julienrf]

The dotty website https://dotty.epfl.ch currently contains several pages that were relevant when Dotty was not yet Scala 3.0.0, especially the section Usage, which is now largely covered by the Getting Started page of the Scala documentation website. Same for the section Contributing, which should go to the Scala 3 Contributing Guide.

When we created the documentation content in the Scala documentation website, we did not immediately remove the corresponding content from the Dotty website, to allow for a smooth transition, but I think it is now time to remove the content that should be hosted on the Scala documentation website instead.

What needs to be done:

• Remove the “Usage” section (if its content is not already covered by the Scala documentation, it should be added there)
• Remove the “Contributing” section (if its content is not already covered by the Scala 3 contributing guide, it should be added there)