Move Container Reg Usage Info to reduce_container_registry_storage.md

What does this MR do and why?

This MR move information about container registry calculations from the the usage quotas page to the reduce container registry page.

Related issue https://gitlab.com/gitlab-org/gitlab/-/issues/423735#note_1625821906

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

This information is moved from https://docs.gitlab.com/ee/user/usage_quotas.html to https://docs.gitlab.com/ee/user/packages/container_registry/reduce_container_registry_storage.html

Screenshot as information was on Usage Quotas page

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.