Fix epic roadmap template labels; add default parent_epics into the template in Developer Evangelism workflow handbook

Why is this change being made?

After creating the 3rd epic copying the handbook epic template string from the new epic roadmap workflow for Developer Evangelism, I've noticed 2 things:

• The scoped labels don't work for the region, and community.
• Manually figuring out which epic (events or content) should be parent is not efficient.

This MR fixes the wrong label examples in the epic description template, and introduces default quick actions for /parent_epic (using events, that's something we do more often), with a commented out quick action for the content epic.

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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