Better onboarding for Vscode users when extension installed

We should store the excalidraw file shown in the demo in the source code so it can easily updated as well.

Also would be great if we can link the sample .excalidraw file when the extension installed for better onboarding under the details section and user can click on it which will open up the file in editor.

Also @pomdtr suggested to show a notification to prompt the user for checking out the sample file as well when installed