Limit SLA to downtime definition

This takes part of
!15942 (comment 2885302949)
and removes the section of the service credits and the pricing that
still needs to be discussed with other stakeholders to be taken into
account with contracts and so on. Leaving only the downtime
definition for now.

This also crosslinks this page from some appropriate places.

Part of https://gitlab.com/gitlab-com/gl-infra/mstaff/-/issues/490