Add documentation page for group-level VSA

Group-level Value Stream Analytics (VSA) was introduced years after
project-level VSA, with documentation that simply pointed to the
original project-level VSA documenation. However, the features have
diverged over time, so now group-level VSA is much more feature-rich.
It deserves its own documentation page now, so this creates it. This
also removes group-level documentation that had been added to the
project-level page.