Tenant Scale handbook: Add capacity planning, estimation with weights, and refinements

Description

This updates the grouptenant scale handbook page defining the process for capacity planning, estimations with weights, and refinements.

Related to refinements, we use the refinement script. For reference, you can see the refinement issues from the Integrations group and how others engineers did refinements following a similar process.

It's an important update, but I want to document the process in a short and simple way. I hope it's clear to everyone!

See live version: https://tenant-scale-capacity-estimations-refinements.about.gitlab-review.app/handbook/engineering/development/enablement/data_stores/tenant-scale/#capacity-planning

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