docs: add timeutil index.md

Author: haithium

docs/src/timeutil/index.md

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+# What is timeutil?
+
+`timeutil` is a C-backed Lua module for wall-clock time, monotonic time, and
+blocking sleep.
+
+It lets Lua read Unix wall-clock timestamps, measure elapsed monotonic time, and
+pause execution for a duration.
+
+## Compatibility
+
+`timeutil` supports:
+
+- Lua 5.1
+- Lua 5.2
+- Lua 5.3
+- Lua 5.4
+- Lua 5.5
+- LuaJIT
+
+## Use Cases
+
+- Get Unix wall-clock time from Lua.
+- Measure elapsed time without wall-clock jumps.
+- Sleep for fractional-second durations.
+- Write timing, polling, timeout, and benchmark code.