hugolib: Do not create paginator pages for the other output formats

This is a recent regression in Hugo, where we have started to produce `/page/30/index.json` when the main output format (usually `HTML`) is set up with pagination.

For JSON this is potentially lot of superflous work and hurts performance.

This commit reinstates the earlier behaviour: We only create paginators if in use in the main output format.

And add a test for it to prevent this from happening again.

Fixes #4890

hugolib/site_render.go

@@ -168,7 +168,8 @@ func pageRenderer(s *Site, pages <-chan *Page, results chan<- error, wg *sync.Wa
 					results <- err
 				}
 
-				if pageOutput.IsNode() {
+				// Only render paginators for the main output format
+				if i == 0 && pageOutput.IsNode() {
 					if err := s.renderPaginator(pageOutput); err != nil {
 						results <- err
 					}