docs: add godoc and bilingual README

doc.go

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+// Package slog provides a small structured logger that can write the same
+// record to text and JSON outputs.
+//
+// A Logger can fan out records to stdout, files, or any external io.Writer.
+// Text writers can include timestamps and a compact traceback frame. JSON
+// writers always emit a structured traceback slice.
+//
+// CreateLogger uses the following defaults:
+//   - prefix: "LOG"
+//   - level: FATAL, which enables info, warn, error, and fatal records
+//   - text timestamps: enabled
+//   - text traceback: disabled
+//   - pretty JSON: disabled
+//
+// Call Level(DEBUG) to enable all records, including debug messages.
+//
+// Basic usage:
+//
+//	logger := slog.CreateLogger().
+//		Prefix("API").
+//		Level(slog.DEBUG).
+//		PrintTraceback(true).
+//		JsonPretty(true)
+//
+//	text := logger.CreateTextStdoutWriter()
+//	jsonFile, err := logger.CreateJsonFileWriter("logs/app.json")
+//	if err != nil {
+//		panic(err)
+//	}
+//
+//	logger.AddWriters(text, jsonFile)
+//	logger.Infoln("service started")
+//
+//	if err := logger.Close(); err != nil {
+//		panic(err)
+//	}
+//
+// Writers created with CreateTextFileWriter or CreateJsonFileWriter are owned by
+// the logger and are closed by Logger.Close. Writers created from existing
+// io.Writer values through CreateTextWriter or CreateJsonWriter remain owned by
+// the caller and are not closed by the logger.
+package slog