Add common prep schema to handbook and consolidate EDM information to EDM Handbook Page

Why is this change being made?

This MR adds the current state of our use of the Common Prep schema to the handbook pursuant to the discovery that was done in this ISSUE.

This MR also consolidates information about the EDM from the dbt guide to the EDM Handbook page. This will allow us to iterate on the EDM 2.0 and Enterprise Data Governance in one place.

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.
  • 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.

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