Add a spinner "Try example directive"

[Carreau]

The first time you click on the "Try example" directive; especially on a slow connection, the iframe can stay blank for quite some time.

It would be good to inject a spinner like the ..jupyterlite directive that says "loading...", to make sure the user is not refreshing or wondering what is happening.

[steppi]

Agreed. It's a good suggestion. I'll add one.