Update Tenant Scale about section on the handbook

Description

Update the grouptenant scale about section on the handbook. I will create different merge requests to improve each section and clarify the process.

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

https://update-tenant-scale-about-handbook.about.gitlab-review.app/handbook/engineering/development/enablement/data_stores/tenant-scale/