Update incident review process re: separate issues

Why is this change being made?

This change reflects the discussion in gitlab-com/gl-infra/production!160 (merged). The primary difference is that, going forward, we'd like to start managing Incident Reviews in separate issues (NOT the same issue where the incident is declared). This change describes the logistics of doing so.

This removes a lot of outdated and redundant information on the incident review handbook page.