docs: Add BYOA GitOps Agent ArgoCD configuration guide

[shlomodaari]

Documents the critical distinction between using label values vs service names when configuring Harness GitOps Agent with existing ArgoCD installations.

Includes:

• How to find correct label values using kubectl commands
• Explanation of "Svc" suffix pattern for service names
• Common mistakes and troubleshooting steps
• Redis secret configuration for BYOA setups

Addresses common connection errors caused by incorrect component naming

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