Handbook oncall updates

In this MR I would like to update our Handbook with more guidance for on-call, with a focus on Support.

Initial impressions:

1. We're missing step by step instructions on what to do after getting paged
2. The info we have is mixed up between roles and is hard to follow: /handbook/on-call
3. Support should have its own handbook page for on-call duties

Resources

• The production team has a page on On-call handovers that could be useful to us: https://about.gitlab.com/handbook/engineering/infrastructure/on-call-handover/
• The secops team has their own on-call guide: https://about.gitlab.com/handbook/engineering/security/secops-oncall.html