resource/page: Add Page.Ancestors

Fixes #10567

resources/page/page.go

@@ -407,6 +407,9 @@ type TreeProvider interface {
 	// To get a section's subsections, see Page's Sections method.
 	Parent() Page
 
+	// Ancestors returns the ancestors of each page
+	Ancestors() Pages
+
 	// Sections returns this section's subsections, if any.
 	// Note that for non-sections, this method will always return an empty list.
 	Sections() Pages

resources/page/page_nop.go

@@ -348,6 +348,10 @@ func (p *nopPage) Parent() Page {
 	return nil
 }
 
+func (p *nopPage) Ancestors() Pages {
+	return nil
+}
+
 func (p *nopPage) Path() string {
 	return ""
 }

resources/page/testhelpers_test.go

@@ -416,6 +416,10 @@ func (p *testPage) Parent() Page {
 	panic("not implemented")
 }
 
+func (p *testPage) Ancestors() Pages {
+	panic("not implemented")
+}
+
 func (p *testPage) Path() string {
 	return p.path
 }