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>

.github/workflows/docs.yml

@@ -0,0 +1,66 @@
+name: Documentation
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: docs
+  cancel-in-progress: true
+
+env:
+  PLAYDATE_SDK_VERSION: "3.1.1"
+
+jobs:
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: maxim-lobanov/setup-xcode@v1
+        with:
+          xcode-version: latest
+
+      - name: Cache Playdate SDK
+        id: sdk-cache
+        uses: actions/cache@v4
+        with:
+          path: ~/PlaydateSDK
+          key: playdate-sdk-${{ env.PLAYDATE_SDK_VERSION }}
+
+      - name: Download Playdate SDK headers
+        if: steps.sdk-cache.outputs.cache-hit != 'true'
+        run: |
+          curl -sL "https://download.panic.com/playdate_sdk/Linux/PlaydateSDK-${PLAYDATE_SDK_VERSION}.tar.gz" | tar xz -C "$HOME"
+          mv "$HOME/PlaydateSDK-${PLAYDATE_SDK_VERSION}" "$HOME/PlaydateSDK"
+          test -f "$HOME/PlaydateSDK/C_API/pd_api.h"
+
+      - name: Install pkg-config module
+        run: |
+          PLAYDATE_SDK_PATH="$HOME/PlaydateSDK" Scripts/install-pkgconfig.sh "$HOME/pkgconfig"
+          echo "PKG_CONFIG_PATH=$HOME/pkgconfig" >> "$GITHUB_ENV"
+
+      - name: Generate documentation
+        run: |
+          swift package --allow-writing-to-directory docs \
+            generate-documentation --target PlayDate \
+            --disable-indexing \
+            --transform-for-static-hosting \
+            --hosting-base-path play-date \
+            --output-path docs
+
+      - uses: actions/upload-pages-artifact@v3
+        with:
+          path: docs
+
+      - id: deployment
+        uses: actions/deploy-pages@v4