[Guardian app] Add documentation explaining how to clean up after uninstalling the application

[soerenbf]

Task description As part of the ceremony performed by guardians through the guardian application, sensitive data is stored on disk on the host machine. This is not automatically removed (at least for macOS and linux) upon uninstallation, thus we should provide documentation on how to do this.

This documentation should be accessible where the user reads about the use of the guardian application.

Is dependant on the implementation of #45

Platform-specific data location

The location used for storing application data is the bundle ID of the application: com.concordium.guardian.., e.g. com.concordium.com.testnet.1234.0. This is prefixed by a platform-specific location:

• Linux: Resolves to $XDG_DATA_HOME or $HOME/.local/share.
• macOS: Resolves to $HOME/Library/Application Support.
• Windows: Resolves to {FOLDERID_RoamingAppData}.

Documentation: https://tauri.app/v1/api/js/path/#appdatadir

[soerenbf]

This is a preliminary solution to #83