Links to another quartodoc (or sphinx) site

[wch]

For example, for shiny, we want to also build a site for htmltools and have links from shiny to htmltools.

[machow]

Note, from Winston, it sounds like the docs might be at rstudio.github.io/py-htmltools (similar to the R version: https://rstudio.github.io/htmltools/)

This should be possible with the interlinks filter: https://machow.github.io/quartodoc/get-started/interlinks.html

TODO: I'll double-check having one quartodoc site, interlink to another (currently, all the doc sites are just pointing to sphinx or mkdocs sites).

[machow]

related to #293