Add feedback examples to Developer Evangelism Mentoring/Coaching handbook

Why is this change being made?

Developer Evangelists engage with coaching and mentoring, and often feedback is shared in private 1:1 conversations. This MR adds an example after getting started with cloud-native from a coffee chat with a Developer Evangelist, now one year later with a promotion and great feedback: https://twitter.com/Didicodes/status/1547337357434753027

In addition, this MR enhances the coaching section with more encouragement and an example thread about helping prevent burnout.

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • 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 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.

---