docs: add comprehensive ACM cluster provider documentation

[macayaven]

Add detailed documentation for the ACM (Advanced Cluster Management) cluster provider feature, including:

• Overview and architecture of ACM multi-cluster support
• Configuration guide for both acm-kubeconfig and acm strategies
• Step-by-step setup instructions with examples
• Prerequisites and RBAC requirements
• Troubleshooting guide for common issues
• Security considerations and best practices
• Production deployment examples

This documentation complements the ACM cluster provider implementation merged in PR #58.

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[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[openshift-ci]

Hi @macayaven. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[macayaven]

Closing in favor of #66 which has a clean commit history and follows Red Hat PR requirements.