Add "Who is using GRIT" section to GitLab docs (html)

Add a section to the GRIT documentation highlighting its adoption across GitLab's ecosystem, including:

• GitLab Dedicated (Hosted Runners service)
• GitLab.com core platform services
• Self-managed customer deployments

This will provide visibility into GRIT's real-world usage and help potential users understand its production readiness.

See gitlab-org/gitlab-runner!5263 (comment 2289854802) for context.