📝 add example-first documentation paths

docs/extend/composition.md

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+# Sink Composition
+
+Presets cover common console and file configurations. Build a `Logger::new(...)` directly when records need routing or multiple outputs.
+
+## Send A Record To Two Destinations
+
+```moonbit
+let logger = @log.Logger::new(
+  @log.fanout_sink(@log.console_sink(), @log.json_console_sink()),
+  min_level=@log.Level::Info,
+  target="service",
+)
+
+logger.info("request complete", fields=[@log.field("status", "200")])
+```
+
+`fanout_sink(...)` writes the same record to every child sink. This keeps human-oriented terminal output and machine-oriented JSON output in one application path.
+
+## Route Warnings Separately
+
+```moonbit
+let logger = @log.Logger::new(
+  @log.split_by_level(
+    @log.callback_sink(fn(rec) {
+      println("alert: \{rec.message}")
+    }),
+    @log.console_sink(),
+    min_level=@log.Level::Warn,
+  ),
+  min_level=@log.Level::Trace,
+  target="service",
+)
+```
+
+Use direct sink composition only when the routing behavior is part of the application design. A preset remains easier to audit when one sink is sufficient.
+
+## API Reference
+
+- [`Logger::new(...)`](../api/logger-new.md)
+- [`fanout_sink(...)`](../api/fanout-sink.md)
+- [`split_by_level(...)`](../api/split-by-level.md)
+- [`callback_sink(...)`](../api/callback-sink.md)