Merge branch 'docs/gb/decoupled-rate-limiting-api' into 'master'

Describe the decoupled limits service in the blueprint

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/98083



Merged-by: Grzegorz Bizon <grzegorz@gitlab.com>
Co-authored-by: Grzegorz Bizon <grzesiek.bizon@gmail.com>