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>

Examples/HelloPlaydate/Sources/HelloPlaydate/Game.swift

@@ -0,0 +1,97 @@
+//
+//  Game.swift
+//  A minimal Playdate game built on the play-date Swift bindings: a
+//  bouncing box, a crank-aimed needle, button logging, and a system menu
+//  item.
+//
+
+import CPlaydate
+import PlayDate
+
+@_cdecl("eventHandler")
+public func eventHandler(
+    pointer: UnsafeMutableRawPointer,
+    event: PDSystemEvent,
+    argument: UInt32
+) -> Int32 {
+    if case .initialize = SystemEvent(event: event, argument: argument) {
+        Playdate.initialize(with: pointer)   // must happen before anything else
+        Game.shared.start()
+    }
+
+    return 0
+}
+
+final class Game {
+    nonisolated(unsafe) static let shared = Game()
+
+    private var x: Float = 200
+    private var y: Float = 120
+    private var dx: Float = 3
+    private var dy: Float = 2
+    private let boxSize = 24
+
+    func start() {
+        Display.setRefreshRate(50)
+
+        System.addMenuItem(title: "reset") { _ in
+            Game.shared.reset()
+        }
+
+        System.setUpdateCallback {
+            Game.shared.update()
+            return true   // redraw the display this frame
+        }
+    }
+
+    private func reset() {
+        x = 200
+        y = 120
+    }
+
+    private func update() {
+        moveBox()
+        handleInput()
+        draw()
+    }
+
+    private func moveBox() {
+        let width = Float(Display.width)
+        let height = Float(Display.height)
+
+        x += dx
+        y += dy
+        if x < 0 || x > width - Float(boxSize) { dx = -dx }
+        if y < 24 || y > height - Float(boxSize) { dy = -dy }
+    }
+
+    private func handleInput() {
+        let (_, pushed, _) = System.buttonState
+        if pushed.contains(.a) {
+            System.log("A pressed at \(System.currentTimeMilliseconds)ms")
+        }
+        if pushed.contains(.b) {
+            (dx, dy) = (-dx, -dy)
+        }
+    }
+
+    private func draw() {
+        Graphics.clear(color: .white)
+        Graphics.drawText("Hëllo from Swift — Ⓑ reverses", x: 8, y: 4)
+        Graphics.fillRect(x: Int(x), y: Int(y), width: boxSize, height: boxSize, color: .black)
+
+        if !System.isCrankDocked {
+            // A needle from the screen center pointing where the crank points.
+            let radians = System.crankAngle * .pi / 180
+            let centerX = Display.width / 2
+            let centerY = Display.height / 2
+            Graphics.drawLine(
+                x1: centerX, y1: centerY,
+                x2: centerX + Int(40 * sinf(radians)),
+                y2: centerY - Int(40 * cosf(radians)),
+                width: 2, color: .xor)
+        }
+
+        System.drawFPS(x: 380, y: 4)
+    }
+}