Add documentation for Tier-1 oncall for Service teams

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This update introduces a new Tier-1 for Service teams section, which explains that individual service teams will now have their own dedicated on-call rotations for alerts related to services they own — helping reduce response times and reinforce code ownership. The Runners Platform rotation is listed as the first active team under this new structure, while the EOC remains in the escalation chain as a backup.

Related issue : gitlab-com/gl-infra/production-engineering#29525 Related Epic : gitlab-com/gl-infra#2067

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Commits

• update doc for tier-1

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