docs: add guide to use physical ethernet connection

docs/ethernet.md

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+# Hard-wired Ethernet Connection
+The Wi-Box's board has an unpopulated header labelled `TXP, TXN, RXP, RXN`. It exposes an un-isolated 100FDX Ethernet interface.
+
+![](img/board.jpg)
+
+## Ethernet Transformer
+Since it appears to go directly to the SOC without protection, an ethernet transformer is **absolutely required** to avoid frying the device when connecting it to your network (think about EMI, lightning, etc.).
+
+To get an ethernet transformer in a user-friendly package, you can use a `IEEE802.3AT PoE splitter`. These isolate the ethernet input with a transformer and:
++ use primary center-taps to extract the common-mode PoE voltage, then feeding the extracted voltage to a DC-DC converter towards the USB connector
++ connecting the secondary of the transformer with the diferential Ethernet data to the output eth cable.
+
+![](img/poe_adapter_listing.webp)
+
+In this case, the `HS1602` is the 100M Eth transformer, but this should be irrelevant.
+![](img/poe_adapter.jpg)
+
+We don't even need to feed it PoE; the transformer will be in-circuit regardless.
+
+So by stripping the output ethernet cable and connecting it to the board, the `eth0` interface will become up:
+
+### Wiring 
+```
+eth | board
+  1 - TXP
+  2 - TXN
+  3 - RXP
+  6 - RXN
+```
+
+> [!NOTE]
+> Keep in mind the header is not the ubiquitous 1.27mm pitch but rather 1mm(?), so that may cause some annoyance if you want to add a header connector (comfortable for re-assembling it into the case).
+
+![Image](img/poe_adapter_ethernet.jpg)
+
+## Default access
+The ethernet interface shows up as `eth0` in the device, and is configured by default to use a static IP address of `192.168.1.10`.
+
+> [!NOTE]
+> Assign your computer's ethernet NIC connected to the device an IP in the same subnet (e.g. `192.168.1.20`) and subnet mask of at least `/24` so it can communicate with the device.
+
+You can connect it directly to a NIC on your computer a it to any IP in the same subnet, and access it via telnet (if the device is stock).
+
+
+## Connecting it to an existing network
+
+To connect it to an existing network, you should either set a static IP in your network or enable DHCP:
+
+### Changing the IP address to a static IP
+For example, to set the IP address to `192.168.20.133`
+```
+ip addr add 192.168.20.133/24 dev eth0
+ip route add default via 192.168.20.1 dev eth0
+```
+
+### Enabling DHCP
+TODOa