doc: community and related software: remove urlextern plugin (archived project)

[nodiscc]

No description provided.

[bttrx]

@nodiscc I don't recommend to remove archived projects from the list. "Archived" just means "unsupported", but not "doesn't work anymore". Maybe someone comes across, forks it, and fixes/improves it. I recommend to just mark the plugin as "unsupported", "outdated", "maintainer needed", whatever.

[nodiscc]

@bttrx the goal of the community and related software section is to provide a list of curated, high quality resources and third party software.

Maybe someone comes across, forks it, and fixes/improves it.

It can still be found through various search engines, github project search, etc. If maintenance resumes, it can be re-added. Projects listed on this page come and go, if we didn't clean it up once in a while it would be cluttered with abandoned projects (which in my experience, never come back to life anyway).