Ignore acm citation link in link check

Produces 403 for the checker, but works manually.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

Author: lsf37

.linkcheck-ignore.yml

@@ -0,0 +1,7 @@
+# Copyright 2025, Proofcraft Pty Ltd
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+urls:
+  # link works, but checker gets 403, because only humans are allowed to check
+  - dl.acm.org/citation.cfm.id=917665