Update single cluster setup with agent support

[yashodgayashan]

Purpose

This pull request updates the single-cluster getting started guide to clarify the roles of agent-based communication between the control plane and data plane, and provides improved instructions for enabling, verifying, and understanding agent mode. The changes make it easier for users to set up secure connectivity and confirm the correct operation of the data plane agent.

Documentation improvements for agent-based communication:

• Added descriptions for cluster-gateway-* and cluster-agent-* pods to clarify their roles in agent-based communication between the control and data planes. [1] [2]
• Updated the data plane registration command to include --enable-agent and --agent-ca-namespace, enabling agent-based communication and specifying the CA namespace.
• Enhanced the explanation of the DataPlane resource to describe agent-based communication and its benefits for secure WebSocket connectivity.
• Added detailed verification steps for checking agent mode status and DataPlane readiness, including commands to inspect the agent.enabled field and Ready condition.

Related Issues

Include any related issues that are resolved by this PR.

Checklist

• Updated sidebars.ts if adding a new documentation page
• Run npm run start to preview the changes locally
• Run npm run build to ensure the build passes without errors
• Verified all links are working (no broken links)