Bump release to 2.0.0 and document release schedule & versioning

Bump release to 2.0.0 and document release schedule & versioning

This bumps the release number of 2.0.0, to reflect the switch to
a new time based release versioning scheme. The downloads page
is updated to describe our policies for release schedules and
release version numbering

The stable release docs are changed to reflect the fact that
the stable version numbers are now just 3 digits long instead
of 4.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>