Documentation: Allow Dedicated customers to customize maximum instance-level CI/CD variable count

Documentation update for:

• Admin UI - https://docs.gitlab.com/ee/administration/settings/continuous_integration.html
• Admin instance limits - https://docs.gitlab.com/ee/administration/instance_limits.html#cicd-variable-limits

Implementation Table

Group	Issue Link
backend	Backend: Allow Dedicated customers to customize maximum instance-level CI/CD variable count
frontend	Allow Dedicated customers to customize maximum instance-level CI/CD variable count
documentation	👈 You are here
Instrumentation	Issue Title