[Keyboard] Add GMMK Numpad (#20067)

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>

keyboards/gmmk/numpad/readme.md

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+# GMMK NUMPAD
+
+![GMMK NUMPAD](https://i.imgur.com/JV4C5os.png)
+
+A 17 Key macropad made and sold by Glorious LLC, equipped with a WB32 microcontroller, with support for a rotary encoder.
+
+* Keyboard Maintainer: [GloriousThrall](https://github.com/GloriousThrall)
+* Hardware Supported: GMMK Numpad
+* Hardware Availability: [GloriousPCGaming.com](https://www.gloriousgaming.com/products/gmmk-numpad)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make gmmk/numpad:default
+
+Flashing example for this keyboard:
+
+    make gmmk/numpad: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 Num key and plug in the keyboard
+* **Keycode in layout**: Press the Num+Del key which is mapped to `QK_BOOT`
+* **Physical switch**: Pinhole located ubnde the zero key press while inserting the USB
+* **From Glorious Core Firmware**: Press Enter+Plus at the same time while plugging in the USB
+
+## Slider
+
+The slider is currently compatible with [Midi2Vol](https://github.com/jesusvallejo/Midi2Vol), [Midi Mixer (Windows)](https://github.com/jpwilliams/midi-mixer-releases/releases), [Midi2Lightroom (Mac)](https://rsjaffe.github.io/MIDI2LR/), and [ControllerMate (Mac)](https://www.orderedbytes.com/controllermate/)
+
+## Encoder Button
+
+The Button is set to KC_CALC by default and is located at 2,3 on the matrix
+
+## Broken
+
+* Bluetooth functionality