docs: add evdev index.md

Author: haithium

docs/src/evdev/index.md

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+# What is evdev?
+
+`evdev` is a Lua module for working with Linux input devices through the evdev
+interface.
+
+It lets Lua discover Linux input devices, read raw events, grab devices, and
+create virtual input devices with uinput.
+
+## Compatibility
+
+`evdev` supports:
+
+- Lua 5.1
+- Lua 5.2
+- Lua 5.3
+- Lua 5.4
+- Lua 5.5
+- LuaJIT
+
+## Use Cases
+
+- Build input automation tools.
+- Read keyboard, mouse, gamepad, and other Linux input events from Lua.
+- Inspect devices under `/dev/input`.
+- Grab a device while handling input yourself.
+- Create virtual keyboards, mice, or controllers for tests and tooling.