parser/pageparser: Fix handling of commented out front matter

When the page parser was rewritten in 0.51, this was interpreted literally, but commented out front matter is used in the wild to "hide it from GitHub", e.g:

```
<!--
+++
title = "hello"
+++
-->
```

Fixes #5478

hugolib/page_test.go

@@ -1586,6 +1586,33 @@ CONTENT:{{ .Content }}
 	)
 }
 
+// https://github.com/gohugoio/hugo/issues/5478
+func TestPageWithCommentedOutFrontMatter(t *testing.T) {
+	b := newTestSitesBuilder(t)
+	b.WithSimpleConfigFile()
+
+	b.WithContent("page.md", `<!--
++++
+title = "hello"
++++
+-->
+This is the content.
+`)
+
+	b.WithTemplatesAdded("layouts/_default/single.html", `
+Title: {{ .Title }}
+Content:{{ .Content }}
+`)
+
+	b.CreateSites().Build(BuildCfg{})
+
+	b.AssertFileContent("public/page/index.html",
+		"Title: hello",
+		"Content:<p>This is the content.</p>",
+	)
+
+}
+
 // TODO(bep) this may be useful for other tests.
 func compareObjects(a interface{}, b interface{}) bool {
 	aStr := strings.Split(fmt.Sprintf("%v", a), "")