add swift docc plugin

Author: lukepistrol

Package.resolved

@@ -13,6 +13,10 @@ let package = Package(
         ),
     ],
     dependencies: [
+        .package(
+            url: "https://github.com/apple/swift-docc-plugin.git",
+            from: "1.0.0"
+        ),
         .package(
             url: "https://github.com/ChimeHQ/SwiftTreeSitter.git",
             exact: "0.7.0"