Update boards on the Tenant Scale handbook page

Description

Another update for grouptenant scale handbook page:

• Update boards. Most helpful for me:
  • Data Stores:Tenant Scale milestone prioritization
  • Data Stores:Tenant Scale milestones
  • Data Stores:Tenant Scale team members
  • Data Stores:Tenant Scale important
• Remove not useful content: hiring, demos, etc.

I will continue with more handbook updates soon 😉

You can see the updated handbook page live https://tenant-scale-boards.about.gitlab-review.app/handbook/engineering/development/enablement/data_stores/tenant-scale/index.html#issue-boards

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