Make Page.Content a method that returns interface{}

To prepare for a `Resource.Content` method.

See #4622

hugolib/embedded_shortcodes_test.go

@@ -69,7 +69,7 @@ func doTestShortcodeCrossrefs(t *testing.T, relative bool) {
 
 	require.Len(t, s.RegularPages, 1)
 
-	output := string(s.RegularPages[0].Content)
+	output := string(s.RegularPages[0].content)
 
 	if !strings.Contains(output, expected) {
 		t.Errorf("Got\n%q\nExpected\n%q", output, expected)