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__meta.go

@@ -821,7 +821,7 @@ func (p *pageMeta) newContentConverter(ps *pageState, markup string) (converter.
 			// This prevents infinite recursion in some cases.
 			return doc
 		}
-		if v, ok := ps.pageOutput.pco.otherOutputs[id]; ok {
+		if v, ok := ps.pageOutput.pco.otherOutputs.Get(id); ok {
 			return v.po.p
 		}
 		return nil