Add rancher-system-agent and delete Node-Agent

[290760647]

• In our Rancher documentation, the section about Node-agent should be updated to the rancher-system-agent component. The node-agent only exists in RKE1 clusters created through Rancher. Since RKE1 has reached EOL and no longer appears by default in rancher 2.12, I recommend updating the architecture diagram, removing the Node-agent content, and adding content about the rancher-system-agent.

[jmeza-xyz]

@290760647

Does this change to the image suffice?