Chore: Add a README for the Node Graph plugin (#54801)

public/app/plugins/panel/nodeGraph/README.md

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+# Node Graph
+
+The node graph visualizes directed graphs or networks. It uses a directed force layout to effectively position the nodes, so it can display complex infrastructure maps, hierarchies, or execution diagrams.
+
+You can read more about the node graph panel <a href="https://grafana.com/docs/grafana/latest/visualizations/node-graph/" target="_blank">here</a>.
+
+<img height="50%" alt="Node graph panel" src="https://grafana.com/static/img/docs/node-graph/node-graph-8-0.png">