docs: add tty index.md

Author: haithium

docs/src/tty/index.md

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+# What is tty?
+
+`tty` provides small cross-platform Lua bindings for terminal inspection.
+
+## Compatibility
+
+`tty` supports:
+
+- Lua 5.1
+- Lua 5.2
+- Lua 5.3
+- Lua 5.4
+- Lua 5.5
+- LuaJIT
+
+## Use Cases
+
+- Detect whether standard streams are attached to a terminal.
+- Check Lua file handles and numeric file descriptors.
+- Read terminal rows and columns for command-line output.
+- Build terminal-aware Lua tools.