Add Principal TW role to chart

Why is this change being made?

This MR updates the career development chart for Engineering to include the Principal Technical Writer role, now that it's been approved and added to the Technical Writer job description page.

Related to #11219 (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
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• Assign reviewers for this change to the correct DRI(s)
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• 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.