hugolib: Reset Page's main output on server rebuilds

Not doing so prevents the paginators to be rebuilt on changes.

Fixes #4819

hugolib/site.go

@@ -1604,6 +1604,7 @@ func (s *Site) resetBuildState() {
 		p.subSections = Pages{}
 		p.parent = nil
 		p.scratch = newScratch()
+		p.mainPageOutput = nil
 	}
 }