Embed of remote Jupyter out-of-band output resolves incorrectly

[agoose77]

Consider this deployment of a MyST site using Jupyter Notebooks:

https://agoose77.github.io/myst-notebook-demo/plot

As of writing, it publishes a plot:

If we embed this into a local MyST server, MyST appears to re-write the embedded output.data.data."image/png".path to resolve against the local content server, which breaks the URL.

From a cursory glance at our source code, I think we're rewriting all Output nodes, even if they're embedded. I don't have an immediate answer as to what to do here, it will take some thought about what embed means with respect to whether it should actually embed the AST or just pull in a placeholder. (I suspect there is an obvious clear answer of the former)