Add FG-4GT-2SX_V2.0

doc/devices/FG-4GT-2SX_V2.0.md

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+# FG-4GT-2SX_V2.0
+
+Following is documentation for unmanaged switch marked as `FG-4GT-2SX_V2.0`.
+
+Original software is running UART on 9600 baud rate.
+
+The signal quality of UART is bad. Therefore, electrical isolation is needed for the TTL wires in some cases. A simple way to isolate them is to pinch them by fingers.
+
+<img src="photos/FG-4GT-2SX_V2.0/electrical_isolation.png" width="300" />
+
+## Brands
+
+* Ruiying RY-4GT-2SX
+
+<img src="photos/FG-4GT-2SX_V2.0/RY-4GT-2SX_label.jpg" width="300" />
+
+## What works
+
+- All four 2.5GBASE-T RJ45 ports at 10/100/1000/2500 Mbps  
+- Both SFP ports supporting 1G, 2.5G and 10G modules 
+- LEDs
+
+## PCB overview
+
+**Board markings**  
+
+- Top silkscreen: FG-4GT-2SX_V2.0
+
+Front panel
+
+<img src="photos/FG-4GT-2SX_V2.0/chassis-front.jpg" width="300" />
+
+Top side
+
+<img src="photos/FG-4GT-2SX_V2.0/PCB-top.jpg" width="300" />
+
+Bottom
+
+<img src="photos/FG-4GT-2SX_V2.0/PCB-bottom.jpg" width="300" />
+
+## Power supply
+
+Input power is delivered via barell plug, `12V 1A` adapter was provided.