Document the launch button functionality

[choldgraf]

This documents the launch button functionality that has been in the myst-theme for a while now. It looks like we never got around to putting it in the actual user documentation.

Preview: https://deploy-preview-2046--mystmd.netlify.app/website-launch-buttons

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Latest commit: cdcc555

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[agoose77]

LGTM! In future, we could likely drop the need for jupyter: true. I think the only issue to be mindful of is to avoid adding the button where people don't want it -- i.e. if opt-out is sufficient.