[Keymap] Add personal keymap to GMMK Pro ANSI (#15314)

Co-authored-by: Drashna Jaelre <drashna@live.com>

keyboards/gmmk/pro/ansi/keymaps/byungyoonc/readme.md

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+byungyoonc's GMMK Pro Layout
+============================
+
+## Keymap
+
+### Base Layer
+![GMMK Pro Base Layer Layout Image](https://i.imgur.com/VvEL08Q.png)
+
+Made a bit of a change on the leftmost side of the keyboard to set Home and End closer to the arrow keys. Also uses a shortcut for Windows Task Manager, Windows PowerToys microphone mute.
+
+### Fn Layer
+![GMMK Pro Fn Layer Layout Image](https://i.imgur.com/uQy2gSh.png)
+
+F-row media functions roughly matches the Keychron keyboard's layout.
+
+Added Debug key for faster debugging iteration.
+
+Supports Secrets input by Fn + Numbers.
+
+The rest are pretty self-explanatory RGB controls, a GUI toggle and an NKRO toggle.
+
+## Rotary Encoder Rotation
+Volume control when no mods, Ctrl+Left/Right when Ctrl is pressed while rotating the encoder.
+
+## RGB Indicators
+Uses side strap RGB as the RGB indicators. 
+- Top third lights on when the Num Lock is turned off.
+- Middle third lights on when the Caps Lock is turned on.
+- Bottom third lights on when the GUI is disabled (via `GUI_TOG`).
+
+## Debounce
+Uses the `DEBOUNCE_TYPE` of `sym_eager_pk` for the shortest response time possible, and `DEBOUNCE` time of `40` in order to eliminate any key chattering.
+
+## NKRO
+N-key rollover is turned on by default.
+
+## RGB Matrix
+Keypresses effects are enabled.
+Disabled default several RGB effects.
+Added custom RGB matrix effect `saturated_solid_multisplash`.
+Every keypress increases the RGB hue value.