Allow to create a shared link manually (for embedding)

From: https://twitter.com/riadbenguella/status/1271392061074350081

I'm interested in embedding your app in my app: https://github.com/dai-shi/remote-faces

It already embeds Excalidraw based on the manually created link: https://howto.excalidraw.com/create-collaboration-session-manually/

If possible, I would like take the same approach for your app.

Nice project!