Add in-section Next/Prev content pointers

Conflicts:
	docs/content/meta/release-notes.md
	docs/content/templates/variables.md

hugolib/page.go

@@ -80,6 +80,8 @@ type PageMeta struct {
 type Position struct {
 	Prev *Page
 	Next *Page
+	PrevInSection *Page
+	NextInSection *Page
 }
 
 type Pages []*Page