Add DocC documentation and a runnable example game

- DocC catalog (Sources/PlayDate/PlayDate.docc) with a curated landing
  page and a Getting Started article; swift-docc-plugin wired up for
  `swift package generate-documentation`, and a GitHub Pages deployment
  workflow renders docs on every push to main.
- Examples/HelloPlaydate: a minimal game (bouncing box, crank needle,
  button handling, system menu item) whose build.sh compiles the game as
  a dylib and produces a runnable simulator .pdx via the SDK's pdc.
  Verified: the pdx builds and exports the eventHandler entry point.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Package.swift

@@ -10,6 +10,10 @@ let package = Package(
             targets: ["PlayDate"]
         ),
     ],
+    dependencies: [
+        // Documentation generation only; not linked into the library.
+        .package(url: "https://github.com/swiftlang/swift-docc-plugin", from: "1.4.0"),
+    ],
     targets: [
         // The Playdate C API headers, resolved through the "playdate"
         // pkg-config module. Run Scripts/install-pkgconfig.sh once to point