[Keymap] Reorganization, cleanup and readmes for drashna code (#15617)

users/drashna/rgb/readme.md

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+# RGB 
+
+Custom RGB code can be disabled by setting `CUSTOM_RGBLIGHT = no` or `CUSTOM_RGB_MATRIX = no` in your `rules.mk`
+
+## RGB Light
+
+### Layer Indication Code 
+
+At least for RGB Light, the `layer_state_set` function is used to detect the current highest layer, and change the underglow based on that layer.  
+
+This works for both the regular layers, and for the default layers, too.
+
+I use the sethsv variants of the commands, so that different modes can be used, as well. 
+
+RGB Matrix uses a custom, per board implementation, at the moment. 
+
+### RGB Light Startup Animation
+
+On startup, if enabled, the board will cycle through the entire hue wheel, starting and ending on the default layer color. 
+
+```c
+void keyboard_post_init_rgb(void) {
+#if defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_STARTUP_ANIMATION)
+    if (userspace_config.rgb_layer_change) { rgblight_enable_noeeprom(); }
+    if (rgblight_config.enable) {
+        layer_state_set_user(layer_state);
+        uint16_t old_hue = rgblight_config.hue;
+        rgblight_mode_noeeprom(RGBLIGHT_MODE_STATIC_LIGHT);
+        for (uint16_t i = 255; i > 0; i--) {
+            rgblight_sethsv_noeeprom( ( i + old_hue) % 255, 255, 255);
+            matrix_scan();
+            wait_ms(10);
+        }
+    }
+#endif
+    layer_state_set_user(layer_state);
+}
+```
+
+This could probably benefit from some cleanup and better handling. 
+
+## RGB Matrix
+
+### Idle Animation
+
+This feature can be toggled with the `RGB_IDL` keycode. 
+
+This sets the mode to the Heatmap Animation when typing, but will switch to the cycle in animations when idle. 
+
+### Layer Indication
+
+This sets the modifier keys to indicate the current layer state, with the option to override the behavior.