Add JTBD label to UX handbook

Why is this change being made?

This MR adds SSOT in the handbook for how to use the Jobs to be Done (JTBD) label.

The UX Department has been working on improving job statement quality. During this process, Product Designers, their managers, and Product Managers work on MRs and issues related to JTBD changes. These changes are currently scattered across the .com project. My proposal is that we have an easy way to find and track JTBD updates/addition, helping teams quickly access all information related to a jtbd and category.

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.

!90165 (closed)

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