docs: CEC.md, which PCs have HDMI-CEC and how to add it

CEC.md

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+# HDMI-CEC: does my PC have it?
+
+The **HDMI-CEC** item in Steamify lets you use Steam with your TV's remote,
+and turns the TV on and off with the PC. It only works when the PC has a
+CEC device (`/dev/cec*`). Many PCs don't: on desktop PCs CEC is rare.
+How it works: [TECHNICAL.md](TECHNICAL.md#hdmi-cec); problems:
+[TROUBLESHOOTING.md](TROUBLESHOOTING.md#the-tv-remote-does-nothing-hdmi-cec).
+
+## Graphics cards
+
+- **NVIDIA:** no. The driver doesn't expose CEC on the HDMI port, on Linux
+  or Windows.
+- **AMD and Intel:** the CEC line of the HDMI port is normally not
+  connected on desktop cards. The Linux drivers (`amdgpu`, `i915`) only
+  support CEC over a **DisplayPort** connection: some DisplayPort-to-HDMI
+  adapters and built-in converter chips pass CEC along ("DP-AUX CEC"). That
+  depends on the adapter or chip, not on the graphics card, so it's worth
+  trying a different adapter.
+
+## Devices with CEC built in
+
+- the **Valve Steam Machine** (Steamify ticks HDMI-CEC by default there);
+- some **Intel NUC** mini PCs, which have a CEC chip on the board;
+- the **Raspberry Pi**.
+
+## Any PC: a USB CEC adapter
+
+A USB CEC adapter, such as the **Pulse-Eight USB-CEC adapter** (or a
+RainShadow Tech one), sits between the PC and the TV on the HDMI cable and
+works on Linux. This is the way to get CEC on a custom build, e.g. with an
+NVIDIA card. Steamify installs what these adapters need
+(`inputattach-cec-units`), so plug it in and turn on HDMI-CEC.
+
+## Check your PC
+
+After turning on HDMI-CEC in Steamify, the menu lists the CEC devices it
+found. Or run:
+
+```bash
+ls /dev/cec*
+```
+
+No device (and no USB adapter): no CEC on this PC. Also turn on CEC on the
+TV itself: it has a brand name there (Sony: BRAVIA Sync, Samsung: Anynet+,
+LG: SimpLink, Philips: EasyLink, Panasonic: VIERA Link).
+
+This overview is based on how the Linux drivers work; hardware differs,
+so the only sure test is `ls /dev/cec*` on your own PC.