Write mdbook summary (#11750)

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

docs/SUMMARY.md

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+# Summary
+
+-   [Introduction](../README.md)
+
+# Customisation
+
+-   [Skinning](skinning.md)
+
+# Deep dive
+
+-   [Cider editor](ciderEditor.md)
+-   [Iconography](icons.md)
+-   [Jitsi](jitsi.md)
+-   [Local echo](local-echo-dev.md)
+-   [Media](media-handling.md)
+-   [Room List Store](room-list-store.md)
+-   [Scrolling](scrolling.md)
+-   [Usercontent](usercontent.md)
+-   [Widget layouts](widget-layouts.md)
+
+# Testing
+
+-   [Cypress end to end](cypress.md)