Add dependabot for antora dependency tracking

Author: mborland

.github/workflows/dependabot.yml

@@ -0,0 +1,10 @@
+# Copyright 2025 Matt Borland
+# Distributed under the Boost Software License, Version 1.0.
+# (See accompanying file LICENSE_1_0.txt or copy at http://boost.org/LICENSE_1_0.txt)
+
+version: 2
+updates:
+  - package-ecosystem: "npm"
+    directory: "/doc"
+    schedule:
+      interval: "weekly"