[docs] classify dart:ffi as vm on docs site, and exclude it for now

closes: #35937
Change-Id: I543a61c4d90877de46b5e3fbc9f2d93c1df821df
Reviewed-on: https://dart-review.googlesource.com/c/93160
Auto-Submit: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>

sdk/lib/ffi/ffi.dart

@@ -2,6 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file
 
+/// {@category VM}
+/// {@nodoc}
 library dart.ffi;
 
 part "native_type.dart";