Add Pixel Rain RGB Matrix effect (#14155)

* Add Pixel animation header file

* Add Pixel animation to documentation

* Add Pixel animation to core include file

* Remove unintended spaces

* Remove seeding of PRNG

* Increase frequency of lit keys

Co-authored-by: filterpaper <filterpaper@localhost>

quantum/rgb_matrix/animations/rgb_matrix_effects.inc

@@ -27,6 +27,7 @@
 #include "hue_pendulum_anim.h"
 #include "hue_wave_anim.h"
 #include "fractal_anim.h"
+#include "pixel_rain_anim.h"
 #include "typing_heatmap_anim.h"
 #include "digital_rain_anim.h"
 #include "solid_reactive_simple_anim.h"