Update actionable_insight template to encourage usage.md

What does this MR do?

Objective

Iterate on the Actionable insights template in gitlab.org/gitlab to hopefully encourage more usage and better standardize 🎯 Actionable Insight issues.

Changes

• Title change,
• Include more markdown hints when creating the issue,
• Tweak some formatting,
• Add an "actionable 😆" task list
• Add a handbook link and cta to create follow-up issues.

Next up

Review and edit the handbook as needed to make sure this still links up properly.