You should be able to use <community.rstudio.com> but I think if you do not add https, a http:// will added for you in the conversion. May not be an issue if redirection from http to https exists on the website.
If you want text and url to be different then you need [community.rstudio.com](https://community.rstudio.com)
More about Links with R Markdown (and Pandoc) : Pandoc - Pandoc User’s Guide
By the way, the extensions you used did not work because Pandoc can't know that without https the string is an URL. It is working on absolute URI only : Pandoc - Pandoc User’s Guide