add excalidraw

service/documentation.md

@@ -977,6 +977,8 @@ We currently use [Graphviz](https://graphviz.org/) to draw charts, but have also
  * [typograms](https://google.github.io/typograms/)
  * [gaphor](https://gaphor.org/) - [not in Debian](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1056299)
  * [wirewiz](https://github.com/wireviz/WireViz) - cabling diagrams
+ * [Excalidraw](https://excalidraw.com/) - has been seen in the wild [in the GitLab issue
+   queue](https://gitlab.torproject.org/tpo/network-health/analysis/-/issues/85#note_3127966_)
 
 TODO: make a section about diagrams, how to make them, why they are
 useful, etc. See [this for inspiration](https://sportebois.medium.com/better-architecture-diagrams-for-agile-teams-actionable-tips-and-lessons-e76627dc4315). Also consider [DRAKON