Support the definition of pre-populated Binder and JupyterHub services

[agoose77]

The launch button (and hopefully, later, thebe) already supports bring-your-own-binder/hub (BYOB) by pasting in the URL of your JupyterHub or BinderHub instance. We should also support the use-case that a site author knows of some pre-existing hubs / binders that can be used with the content (e.g. a hub that has the necessary data-files available).

Adding support for this involves extending the launch button to display these launch targets, and pulling this information from a standardised place, such as the MyST frontmatter.