Fix auto generated header ids so they don't contain e.g. hyperlink destinations (note)

This makes the header ids match the newly added dt ids.

Also make sure newlines are preserved in hooks' `.PlainText`.

Fixes #13405
Fixes #13410

hugolib/content_render_hooks_test.go

@@ -350,3 +350,31 @@ Image: ![alt-"<>&](/destination-"<> 'title-"<>&')
 		})
 	}
 }
+
+// Issue 13410.
+func TestRenderHooksMultilineTitlePlainText(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- content/p1.md --
+---
+title: "p1"
+---
+
+First line.
+Second line.
+----------------
+-- layouts/_default/_markup/render-heading.html --
+Plain text: {{ .PlainText }}|Text: {{ .Text }}|
+-- layouts/_default/single.html --
+Content: {{ .Content}}|
+}
+`
+	b := Test(t, files)
+
+	b.AssertFileContent("public/p1/index.html",
+		"Content: Plain text: First line.\nSecond line.|",
+		"|Text: First line.\nSecond line.||\n",
+	)
+}