Refresh DR WG page, set exit criteria

Why is this change being made?

The DR WG page is out of date, in two key ways:

1. The RTO/RPO targets are referenced on a page which no longer exists. (Was the old PM proposal, which was confusing people)
2. Added an incremental cost target
3. Updated membership as well as refined definitions of RTO/RPO.

Exit criteria

Exit criteria / targets as the following:

1. RTO of 2 hours. We have a 99.95% availability target which allows for 1hr of downtime per quarter, and 4 hours per year.
2. RPO of 1 hour. This is based on what I believe was the target set by executive leadership.
3. Incremental cloud cost of 20%.

Membership

Updated Product DRI to be @joshlambert, and set @joergheilig as executive sponsor.

Author Checklist

• Provided a concise title for this Merge Request (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, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • 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 on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• 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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