users-guide: Ensure extlinks is compatible with Sphinx <4

The semantics of the extlinks attribute annoyingly changed in Sphinx 4 which we addressed in #22690 (closed). However, in so doing we lost compatibility with older Sphinx versions, which are still in use on some distributions. Fix this.

However, the version check is a bit complicated due to #23818 (closed), which I also fix.

Fixes #23807 (closed). Fixes #23818 (closed).