Add ability to mark yourself as a mentor

Why is this change being made?

We had some discussions in the Generational Understanding TMRG about how to find mentors easily and thought it would be nice to be able to self identify as being open to mentoring others. The team page seemed like a good spot to start, so I added a new entry to the person yaml that allows people to see if you are open to mentoring.

The Available as a mentor text will link to this handbook page: https://about.gitlab.com/handbook/engineering/career-development/mentoring/

Thoughts for future enhancements to link to (or list out) available mentors from this page: https://about.gitlab.com/handbook/engineering/career-development/mentoring/

Before

After

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
• Assign this change to the correct DRI
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  • 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.
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