Update Tenant Scale handbook with retrospectives

Description

This update the grouptenant scale handbook page with the retrospective process.

• I've granted access to everyone, see direct membership.
• I updated the async retrospective project gitlab-org/async-retrospectives!733 (merged)

Live demo https://tenant-scale-retrospectives.about.gitlab-review.app/handbook/engineering/development/enablement/data_stores/tenant-scale/index.html#retrospectives

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/391867.