Add swift-docc-plugin as a dependency

Author: hoseiocean

Package.swift

@@ -16,6 +16,9 @@ let package = Package(
       targets: ["DataStructuresKit"]
     ),
   ],
+  dependencies: [
+    .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.4.3"),
+  ],
   targets: [
     .target(
       name: "DataStructuresKit",