Deprecate data/events.yml and add new location/handbook

Why is this change being made?

MRs against the old source in data/events.yml do not work anymore, for example !112152 (diffs) You won't notice the problem that the review app does not render it, and keep looking. A Slack thread (internal) brought insights into the migration to the digital-experience/buyer-exoerience repository in gitlab-com/marketing/digital-experience/buyer-experience#953 (closed)

To raise awareness for everyone who navigates directly into the Git repository and does not use the /events - scroll down - edit source flow, this MR adds a deprecation note to the old yaml file, including a URL to the new location and handbook instructions.

Suggest posting to #whats-happening-at-gitlab or newsletter to increase awareness.

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.
• 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
• 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.

---