docs: adds Retrospective section to Epic Management
Adds a section to Epic Management called "Retrospective", with a link to the proposed epic retro template.
This change elaborates on the final bullet point under "Epic Checklist" in the Epic description template.
See FAL-3121 for some context.
Testing instructions
- Read through the Epic Management > Retrospective text added by this change, and ensure it is accurate and formatted correctly.
- Read through the proposed epic retro template and comment if anything should be added/changed.