add doc for STEAMEMO_IG204_V1

doc/devices/STEAMEMO_IG204_V1.md

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+# Steamemo IG204-V1
+
+Following is documentation for unmanaged switch marked as `IG204-V1`.
+
+Using SPI clamp in-board is the only method for initial installation.
+
+### Label specifications
+
+- **Name**: 2.5G Ethernet Switch  
+- **Model**: IG204 V1
+- **Ports**:  
+  - 4 × RJ45: 10/100/1000/2500 Mbps  
+  - 2 × SFP: 1000 / 2500 / 10000 Mbps  
+
+### What works (expected from label + similar devices)
+
+- Four 2.5GBASE-T RJ45 ports at 10/100/1000/2500 Mbps  
+- Two SFP ports supporting 1G, 2.5G and 10G modules 
+- LEDs
+
+### PCB overview
+
+**Board markings**  
+- Top silkscreen: PB-2131
+
+
+Top side
+
+<img src="photos/STEAMEMO_IG204_V1/PCB-top.jpg" width="600" />
+
+### Connectors
+### T7, serial console
+
+| `T7` pin | Signal      |
+| -------- | ----------- |
+| 1        | TX (Output) |
+| 2        | GND         |
+| 3        | RX (Input)  |
+| 4        | 3V3         |
+
+### Power supply
+
+Input power is delivered via barell plug, `12V 1A` adapter was provided.