Improve guidance and references for corrective actions

Why is this change being made?

An escalation from https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/23654#note_1391301346 corrective action resulted in the priority of the corrective action being reduced and there was some confusion about should this have taken place. This MR clarifies that severity should match the incident (not changed) and that priority can be decided based on https://about.gitlab.com/handbook/engineering/infrastructure/team/reliability/issues.html#issue-priority but should be based on the severity if no other information is available.

I also added some references to make it easier to create corrective actions directly from the handbook page.

Author Checklist

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