added docc plugin

Author: leogdion

Package.resolved

@@ -24,7 +24,8 @@ let package = Package(
     ),
   ],
   dependencies: [
-    .package(url: "https://github.com/apple/swift-syntax.git", from: "601.0.1")
+    .package(url: "https://github.com/apple/swift-syntax.git", from: "601.0.1"),
+    .package(url: "https://github.com/swiftlang/swift-docc-plugin", from: "1.4.0")
   ],
   targets: [
     .target(