Add hiring note about internal candidates

Why is this change being made?

The Greenhouse process is tailored towards external candidates, and I had some questions about the process for internal candidates. The change here reflects what I learned and I just wanted to provide a short update to clarify for others.

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.

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