docs: add comprehensive ACM cluster provider documentation

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 openshift#58.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: macayaven