Update documentation for rgb matrix (#3023)

* Update documentation for rgb matrix

- Updated keycodes.md to include RGB matrix keycodes + necessary link

docs/keycodes.md

@@ -284,6 +284,22 @@ This is a reference only. Each group of keys links to the page documenting their
 |`RGB_MODE_XMAS`    |`RGB_M_X` |Christmas animation mode                                            |
 |`RGB_MODE_GRADIENT`|`RGB_M_G` |Static gradient animation mode                                      |
 
+## [RGB Matrix Lighting](feature_rgb_matrix.md)
+
+|Key                |Aliases   |Description                                                         |
+|-------------------|----------|--------------------------------------------------------------------|
+|`RGB_TOG`          |          |Toggle RGB lighting on or off                                       |
+|`RGB_MODE_FORWARD` |`RGB_MOD` |Cycle through modes, reverse direction when Shift is held           |
+|`RGB_MODE_REVERSE` |`RGB_RMOD`|Cycle through modes in reverse, forward direction when Shift is held|
+|`RGB_HUI`          |          |Increase hue                                                        |
+|`RGB_HUD`          |          |Decrease hue                                                        |
+|`RGB_SAI`          |          |Increase saturation                                                 |
+|`RGB_SAD`          |          |Decrease saturation                                                 |
+|`RGB_VAI`          |          |Increase value (brightness)                                         |
+|`RGB_VAD`          |          |Decrease value (brightness)                                         |
+|`RGB_SPI`          |          |Increase effect speed (does no support eeprom yet)                  |
+|`RGB_SPD`          |          |Decrease effect speed (does no support eeprom yet)                  |
+
 ## [Thermal Printer](feature_thermal_printer.md)
 
 |Key        |Description                             |