Add DocC plugin to PuzzleKit

This will allow for building the docs in Actions.

Package.resolved

@@ -0,0 +1,24 @@
+{
+  "originHash" : "6230822fda4d48a6effa9ee9a66ef003d919801cc84bb804a93e1bcc9b6817c8",
+  "pins" : [
+    {
+      "identity" : "swift-docc-plugin",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/apple/swift-docc-plugin.git",
+      "state" : {
+        "revision" : "85e4bb4e1cd62cec64a4b8e769dcefdf0c5b9d64",
+        "version" : "1.4.3"
+      }
+    },
+    {
+      "identity" : "swift-docc-symbolkit",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/swiftlang/swift-docc-symbolkit",
+      "state" : {
+        "revision" : "b45d1f2ed151d057b54504d653e0da5552844e34",
+        "version" : "1.0.0"
+      }
+    }
+  ],
+  "version" : 3
+}

Package.swift

@@ -12,6 +12,9 @@ let package = Package(
             name: "PuzzleKit",
             targets: ["PuzzleKit"]),
     ],
+    dependencies: [
+        .package(url: "https://github.com/apple/swift-docc-plugin.git", from: "1.0.0"),
+    ],
     targets: [
         // Targets are the basic building blocks of a package, defining a module or a test suite.
         // Targets can depend on other targets in this package and products from dependencies.