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/build.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+#
+# Builds HelloPlaydate.pdx for the Playdate Simulator: compiles the game as
+# a dylib, places it in the pdc source folder, and runs the SDK's pdc.
+#
+# Device builds need the Embedded Swift + ARM toolchain setup described in
+# the repository README; this script covers the simulator only.
+
+set -eu
+cd "$(dirname "$0")"
+
+sdk_path="${PLAYDATE_SDK_PATH:-$HOME/Developer/PlaydateSDK}"
+
+swift build -c release
+bin_path="$(swift build -c release --show-bin-path)"
+cp "$bin_path/libpdex.dylib" Source/pdex.dylib
+
+"$sdk_path/bin/pdc" Source HelloPlaydate.pdx
+
+echo "Built HelloPlaydate.pdx — run it with:"
+echo "  open -a \"$sdk_path/bin/Playdate Simulator.app\" HelloPlaydate.pdx"