docs: adding install instructions for app WIP

docs/install.md

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+# Installing and Running Docable Notebooks Tray app
+
+Download and run the installer for your operating system from the links below:
+
+- [Windows]()
+- [macOS]()
+- [Linux]()
+
+After running the installer, you should be able to find the docable-tray app icon in your application list:
+
+# Windows
+
+Run the docable-tray app to start Docable Notebooks server in the background. 
+
+![docable tray windows app](img/docable-tray-win.png)
+
+You can confirm the Docable Notebook has started successfully, by righ-clicking on the Docable icon in Windows Taskbar and clicking on "Open Docable...".
+
+![docable tray windows app running](img/docable-tray-win-run.gif)
+
+Your default browser should now open `http://localhost:3000/`.
+
+# macOS
+
+TODO.
+
+
+# Updating
+
+To update the docable-tray app, download and run the latest installers from [installation section]() above.