Draft: Add steps to the documented DRP handbook page

Why is this change being made?

Results of the FY22 annual business continuity test identified the need for activation of the business continuity plan as a result of DRP use and a retrospective process be added to the DRP after each occurrence. This is to help comply with our BCD-04 - Contingency Plan Testing & Exercises security control

Additionally, removed the steps related to disaster slack channel, global disaster alert and google public alert websites as all have been deprecated or no longer used.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
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• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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