Restructure the starting pages

[OsirisTerje]

This page: https://docs.nunit.org/ doubles up what is here: https://docs.nunit.org/articles/nunit/intro.html

Can we move the 2nd to be the 1st ?

And then make the text there more informative?

If you are new to NUnit, we suggest you begin by reading the Getting Started section of this site. Those who have used earlier releases may want to begin with the Upgrading section.

In the NUnit Introduction, which could be the new starting page imho, it refers to the Upgrading section, that don't exist on the same level as Get Started, but is a sub-section of the same. And it is of course outdated and badly informed.