Top level heading for common config (RGB Matrix) (#13120)

* Top level heading for common config

Prior to this, the some of the common config looks like a detail of the APA102 driver

* Change heading to Common Config (RGB Matrix)

docs/feature_rgb_matrix.md

@@ -229,6 +229,8 @@ Configure the hardware via your `config.h`:
 
 ---
 
+## Common Configuration :id=common-configuration
+
 From this point forward the configuration is the same for all the drivers. The `led_config_t` struct provides a key electrical matrix to led index lookup table, what the physical position of each LED is on the board, and what type of key or usage the LED if the LED represents. Here is a brief example:
 
 ```c