Adding the new Severity SLOs for Corrective Actions to sites/handbook/source/handbook/engineering/infrastructure/incident-review/index.html.md

Why is this change being made?

Adding the new Severity SLOs for Corrective Actions to our CA paragraph in the Incident Review page, coming from this Q/A department work/comment: https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10516#note_375948861