[Fleet] Add Fleet diagrams to dev_docs (#110794)

* Add initial fleet dev diagrams to dev_docs

* Move Fleet Server arrow to ES instead of Kibana

x-pack/plugins/fleet/dev_docs/diagrams/README.md

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+# Fleet Diagrams
+
+This directory contains diagrams for various components and concepts around the Fleet application. Diagrams are created using https://excalidraw.com/. To edit a diagram, open the `.excalidraw` file in the Excalidraw web application, then save the file and overwrite that same `.excalidraw` file. Also, be sure to export your diagram as a `.png` and update the image version of the diagram you're editing. 
+
+Excalidraw is an open source diagramming tool for creating simple, effective diagrams and illustrations. To learn more about it, check out their GitHub repository: https://github.com/excalidraw/excalidraw.