Add Incubation Department Career Development matrix

Why is this change being made?

The Incubation Department is maturing so it'd be great for folks to see what the career development looks like like we do for engineering departments.

Additionally the Principal Incubation Engineer role was missing from the handbook so I've filled in a description based on the Principal Engineer role's description.

Here's a placeholder for the incubation department matrix:

Here's the flow diagram being added to the top level engineering career development page:

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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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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