Create buyer experience handbook page

Why is this change being made?

This MR is a first draft of the handbook page for the Buyer Experience repository with information on:

1. The Overview: Purpose, Vision, Mission, Objectives, Strategy
2. Content Editing Experience: Where content exists and how to update
3. Developer Experience: Where documentation exists and how it works with www-gitlab-com

Review App link to new Buyer Experience page

Closes gitlab-com/marketing/digital-experience/buyer-experience#266 (closed)

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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