Add usb_hid keyboard example and link in docs

Adds examples/usb_hid_keyboard.py with a complete USB HID keyboard
demo for Raspberry Pi Zero (W), including prerequisites, wiring, and
usage instructions in the docstring. Links the example in
docs/examples.rst.

Closes #628

Author: makermelissa-ai-assistant

docs/examples.rst

@@ -35,3 +35,7 @@ See the `CircuitPython docs <https://circuitpython.readthedocs.io/>`_ for extens
 .. literalinclude:: ../examples/piblinka.py
     :caption: examples/piblinka.py
     :linenos:
+
+.. literalinclude:: ../examples/usb_hid_keyboard.py
+    :caption: examples/usb_hid_keyboard.py
+    :linenos:

examples/usb_hid_keyboard.py

@@ -0,0 +1,103 @@
+# SPDX-FileCopyrightText: 2023 Björn Bösel for Adafruit Industries
+#
+# SPDX-License-Identifier: MIT
+
+"""
+USB HID Keyboard Example for Raspberry Pi Zero (W)
+===================================================
+
+Demonstrates how to use Blinka's ``usb_hid`` module to turn a
+Raspberry Pi Zero into a USB HID keyboard.
+
+Prerequisites
+-------------
+1. Enable the dwc2 overlay (once, then reboot)::
+
+       sudo bash -c "echo 'dtoverlay=dwc2' >> /boot/config.txt"
+       sudo reboot
+
+2. Load the libcomposite kernel module::
+
+       sudo modprobe libcomposite
+
+   To make it persistent across reboots::
+
+       sudo bash -c "echo 'libcomposite' >> /etc/modules"
+
+3. Install the CircuitPython HID library::
+
+       pip3 install adafruit-circuitpython-hid
+
+Wiring
+------
+- Wire a button between GP20 and GND (types Shift+A).
+- Wire a button between GP21 and GND (types "Hello World!").
+- Wire an LED + 1k resistor between GP16 and GND.
+- Connect the Pi Zero to the host computer via USB.
+
+Run with::
+
+    sudo -E python3 usb_hid_keyboard.py
+
+The ``-E`` flag preserves the user environment so pip-installed
+packages are found when running as root.
+"""
+
+import time
+
+from adafruit_hid.keyboard import Keyboard
+from adafruit_hid.keyboard_layout_us import KeyboardLayoutUS
+from adafruit_hid.keycode import Keycode
+
+import board
+import digitalio
+import usb_hid
+from usb_hid import Device
+
+# Button pins (activate internal pull-ups)
+keypress_pins = [board.D20, board.D21]
+
+# What each button sends: a Keycode or a string
+keys_pressed = [Keycode.A, "Hello World!\n"]
+control_key = Keycode.SHIFT
+
+# Set up button inputs with pull-ups
+key_pin_array = []
+for pin in keypress_pins:
+    key_pin = digitalio.DigitalInOut(pin)
+    key_pin.direction = digitalio.Direction.INPUT
+    key_pin.pull = digitalio.Pull.UP
+    key_pin_array.append(key_pin)
+
+# Set up LED output
+led = digitalio.DigitalInOut(board.D16)
+led.direction = digitalio.Direction.OUTPUT
+
+# Create the USB HID keyboard
+usb_hid.enable([Device.KEYBOARD], boot_device=0)
+keyboard = Keyboard(usb_hid.devices)
+keyboard_layout = KeyboardLayoutUS(keyboard)
+
+print("Waiting for key press...")
+
+while True:
+    for key_pin in key_pin_array:
+        if not key_pin.value:  # Button pressed (grounded)
+            i = key_pin_array.index(key_pin)
+            print("Pin #%d is grounded." % i)
+
+            led.value = True
+
+            while not key_pin.value:
+                pass  # Wait for release
+
+            key = keys_pressed[i]
+            if isinstance(key, str):
+                keyboard_layout.write(key)
+            else:
+                keyboard.press(control_key, key)
+                keyboard.release_all()
+
+            led.value = False
+
+    time.sleep(0.01)