The features added in each major version is also unlikely the first things
someone wants to know about Mesa. So let's move this into the
versions.rst article.

This documentation is severely out of date anyway, and as it doesn't
seem like anyone is interested in documenting this any more, we should
probably consider driopping versions.rst entirely in the longer run.

But for now, this makes the front-page much more approachable.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <!7709>