General RGB matrix fixes (#2931)

General RGB matrix fixes (#2931)

* Added Modular keyboards L,R and NUM

Created code modules for the 3 modules of the modular keyboard.
Original idea by MechboardsUK. Uses i2c implementation similar to lets
split

* Remove modular from master

This is to fix incorrect branching

* General fixes for RGB_matrix

- Complited speed support for all effects
- Fixed raindrop effects to initialized after toggle
- Fixed raindrop effects to use all available LEDs
- Fixed effect step reverse function
- Moved RGB_MATRIX_SOLID_REACTIVE under correct flag

* Documentation update for RGBmatrix

* More doc updates