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>

Sources/PlayDate/PlayDate.docc/GettingStarted.md

@@ -0,0 +1,66 @@
+# Getting Started
+
+Bootstrap the bindings from your game's entry point and drive a frame loop.
+
+## Overview
+
+A Playdate game has a single C entry point, `eventHandler`, which the
+firmware calls with a `PlaydateAPI*` and an event code. Export it with
+`@_cdecl`, call ``Playdate/initialize(with:)`` on the first event, and
+install an update callback:
+
+```swift
+import CPlaydate
+import PlayDate
+
+@_cdecl("eventHandler")
+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()
+
+    func start() {
+        Display.setRefreshRate(50)
+
+        System.setUpdateCallback {
+            self.update()
+            return true   // true = redraw the display this frame
+        }
+    }
+
+    func update() {
+        let (_, pushed, _) = System.buttonState
+        if pushed.contains(.a) {
+            System.log("A pressed")
+        }
+
+        Graphics.clear(color: .white)
+        Graphics.drawText("Hello, Playdate", x: 8, y: 8)
+        System.drawFPS()
+    }
+}
+```
+
+## Conventions to know
+
+- **Initialization.** Calling any wrapper before
+  ``Playdate/initialize(with:)`` is a programmer error and will crash.
+- **Errors.** Fallible operations use typed throws — ``PlaydateError``
+  generally, ``Network/NetError`` for network I/O.
+- **Ownership.** A wrapper that creates a C object frees it on `deinit`;
+  keep the wrapper referenced for as long as you use it. Wrappers vending
+  OS-owned objects don't free them — keep the owner alive instead, as
+  documented on each API.
+- **Threading.** The Playdate runtime is single-threaded; don't call the
+  API from other threads.