Improve landing page

[OsirisTerje]

The landing page could need some updates.
We don't need to give the legacy documentation that much focus now.
The documentation is now focused around how we have built NUnit, and less about how to use NUnit. We should focus on the latter, but still provide good visible links to the reference sections, the developer details and other stuff that could be usable to contributors and developers.

• Move legacy documentation into its own sub-page Move legacy docs from main page to sidebar #809
• Add a section for announcements
• Add a section for tutorials, with "How to" information
• Add information on where to find different stuff around NUnit, the repos we have, Microsoft/dotnet related information and so on

This can be just section headers with links to subpages, or some other nice way of getting a good landing page.