[ENH] iframe revamp

[lmeyerov]

Create explicit iframe generation helper, and associated cleanup, as it frequently comes up in integrations:

• add allow=fullscreen to existing allow list
• expose as part of plotter and highlight in docs

[lmeyerov]

Addition: also a link form. Maybe switch render to:

Plotter.plot(render: bool | 'link' | 'subpath' | 'iframe' | 'jupyter' | 'streamlit' | ...)