hugolib: Consider summary in front matter for .Summary

Add the ability to have a `summary` page variable that overrides
the auto-generated summary.  Logic for obtaining summary becomes:

  * if summary divider is present in content, use the text above it
  * if summary variables is present in page metadata, use that
  * auto-generate summary from first _x_ words of the content

Fixes #5800

hugolib/page__meta.go

@@ -64,6 +64,8 @@ type pageMeta struct {
 	title     string
 	linkTitle string
 
+	summary string
+
 	resourcePath string
 
 	weight int
@@ -361,6 +363,9 @@ func (pm *pageMeta) setMetadata(p *pageState, frontmatter map[string]interface{}
 		case "linktitle":
 			pm.linkTitle = cast.ToString(v)
 			pm.params[loki] = pm.linkTitle
+		case "summary":
+			pm.summary = cast.ToString(v)
+			pm.params[loki] = pm.summary
 		case "description":
 			pm.description = cast.ToString(v)
 			pm.params[loki] = pm.description