render rextendr rd files here

To minimize duplicating documentation and sites that need to be tracked, we should consider collecting rextendr's rd files here, converting them to qmd with rd2qmd, and then pushing those through the extendr.rs build process. This would allow us to remove pkgdown from ci/cd in the rextendr repo and also make it easier to apply consistent styling.

The inspiration for this can be found here: https://github.com/R-ArcGIS/r-bridge-site.