Add Tenant Scale Sisense dashboard

Description

Add the new Sisense dashboard↗ for the group and update the engineering metric handbook pages.

Live pages:

• https://add-tenant-scale-sisense-dashboard.about.gitlab-review.app/handbook/engineering/development/enablement/data_stores/tenant-scale/#dashboards
• https://add-tenant-scale-sisense-dashboard.about.gitlab-review.app/handbook/engineering/metrics/enablement/data-stores/tenant-scale/

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