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.

🤖 Generated with Claude Code

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: macayaven
Once this PR has been reviewed and has the lgtm label, please assign cali0707 for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[macayaven]

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