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-shed.yaml

@@ -1,4 +1,14 @@
 ---
+# Hardware: M5Stack ATOM Lite + ATOM Hub Switch
+# Pinout:
+#   GPIO19                             -> Light (switch gpio)
+#   GPIO21                             -> i2c (i2c scl)
+#   GPIO22                             -> Garden Light (switch gpio)
+#   GPIO25                             -> i2c (i2c sda)
+#   GPIO26  Grove SDA
+#   GPIO32  Grove SCL
+# I2C:
+#   0x27  pca9554_module (pca9554)
 substitutions:
   device: shed
   name: Shed