Add Page.Contents with scope support

Note that this also adds a new `.ContentWithoutSummary` method, and to do that we had to unify the different summary types:

Both `auto` and `manual` now returns HTML. Before this commit, `auto` would return plain text. This could be considered to be a slightly breaking change, but for the better: Now you can treat the `.Summary` the same without thinking about where it comes from, and if you want plain text, pipe it into `{{ .Summary | plainify }}`.

Fixes #8680
Fixes #12761
Fixes #12778
Fixes #716

hugolib/page__output.go

@@ -65,6 +65,7 @@ func newPageOutput(
 		p:                       ps,
 		f:                       f,
 		pagePerOutputProviders:  providers,
+		MarkupProvider:          page.NopPage,
 		ContentProvider:         page.NopPage,
 		PageRenderProvider:      page.NopPage,
 		TableOfContentsProvider: page.NopPage,
@@ -95,6 +96,7 @@ type pageOutput struct {
 	// output format.
 	contentRenderer page.ContentRenderer
 	pagePerOutputProviders
+	page.MarkupProvider
 	page.ContentProvider
 	page.PageRenderProvider
 	page.TableOfContentsProvider
@@ -119,7 +121,7 @@ func (po *pageOutput) isRendered() bool {
 	if po.renderState > 0 {
 		return true
 	}
-	if po.pco != nil && po.pco.contentRendered {
+	if po.pco != nil && po.pco.contentRendered.Load() {
 		return true
 	}
 	return false
@@ -139,6 +141,7 @@ func (p *pageOutput) setContentProvider(cp *pageContentOutput) {
 	}
 	p.contentRenderer = cp
 	p.ContentProvider = cp
+	p.MarkupProvider = cp
 	p.PageRenderProvider = cp
 	p.TableOfContentsProvider = cp
 	p.RenderShortcodesProvider = cp