[Keyboard] Add Snatchpad keyboard (#16330)

Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Ryan <fauxpark@gmail.com>
Co-authored-by: xia0 <xiao@morgan.net.au>

keyboards/handwired/snatchpad/readme.md

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+# snatchpad
+
+![snatchpad etch-a-sketch macropad](https://i.imgur.com/aRIM8Nah.jpg)
+
+A 6-key macropad with 2 rotary encoders intended for use as a pointing device.
+
+In addition to the 3d printed parts and microcontroller, the following hardware is required:
+* 4 * 3x3 mm threaded inserts
+* 4 * 4mm M3 bolt
+* USB daughter board
+
+---
+
+* Keyboard Maintainer: [xia0](https://github.com/xia0)
+* Hardware Supported: e.g. Arduino Pro Micro
+* Hardware Availability: [github](https://github.com/xia0/keeb_files/tree/main/snatchpad)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make handwired/snatchpad:default
+
+Flashing example for this keyboard:
+
+    make handwired/snatchpad:default:flash
+
+See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).
+
+## Bootloader
+
+Enter the bootloader in 3 ways:
+
+* **Bootmagic reset**: Hold down the key at (0,0) in the matrix (usually the top left key or Escape) and plug in the keyboard
+* **Physical reset button**: Briefly press the button on the back of the PCB - some may have pads you must short instead
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available