Add instructions on generating a webpage for a notebook that depends on packages in a custom/local registry

[mtfishman]

Thanks for the nice package and your work on Pluto.jl.

It would be nice to add instructions for using this package to generate a webpage for a notebook that depends on packages that are only available in a custom/local registry, rather than Julia's General registry. As expected, if you naively use this template for that use case, it can't find where to install the packages.

See my use case here: https://github.com/ITensor/itensor-deconstructed, that depends on packages in this custom/local registry: https://github.com/ITensor/ITensorRegistry.

My naive fix was adding this line to the workflow to add the custom/local registry: https://github.com/ITensor/itensor-deconstructed/blob/6fdc33d1be79e4dcb639862b70159563b88aa470/.github/workflows/ExportPluto.yaml#L49, which is able to successfully generate this webpage: https://itensor.github.io/itensor-deconstructed/itensor-deconstructed.html. Maybe there is a better way to handle this use case, if so please let me know!

I'm happy to make a PR with instructions, but partially I'm raising this issue as a reminder and also a hint for other users who are interested in doing something similar.