Update retrospective
Updates to the handbook in line with #3254 (closed).
- Add timing to agenda of retrospective to ensure we have plenty of room for discussion
- Clarify the purpose/audience of the retrospective
- Be explicit about the need for Engineering Managers to collaborate and identify similarities between their team retrospectives
@dhavens since @edjdev is out, would you mind reviewing + merging if all looks good to you?
/cc @gitlab-com/backend-managers @timzallmann @sarrahvesselov @meks