Document hardware, pinout and I2C devices in a header comment of every device

Merge the old pinout comment blocks into the header and mark what each pin is used for.

mcu-echo-1.yaml

@@ -1,4 +1,14 @@
 ---
+# Hardware: M5Stack ATOM Echo
+# Pinout:
+#   GPIO19                             -> speaker_bus (i2s_audio i2s_bclk_pin)
+#   GPIO22                             -> speaker_id (speaker i2s_audio i2s_dout_pin)
+#   GPIO23                             -> echo_microphone (microphone i2s_audio i2s_din_pin)
+#   GPIO26  Grove SDA
+#   GPIO27                             -> LED (light esp32_rmt_led_strip)
+#   GPIO32  Grove SCL
+#   GPIO33                             -> speaker_bus (i2s_audio i2s_lrclk_pin)
+#   GPIO39                             -> Button (binary_sensor gpio) (inverted)
 substitutions:
   device: echo-1
   name: Echo 1