Add page for collecting coaching and feedback resources

Why is this change being made?

💡 Provide a detailed answer to the question on why this change is being proposed, in accordance with our value of Transparency.

This MR adds a new handbook page to collect coaching and feedback resources for managers. This is being done as part of the Elevate program for Sam & Steve's capstone project.

Review app: https://stkerr-elevate-project.about.gitlab-review.app/handbook/people-group/learning-and-development/manager-coaching-and-leadership-resources/

---