[Java][Docs] Compare/contrast the Netty and Unsafe memory backends

We should compare why you might want to use each.

Are there benchmarks in the Java benchmark suite that might also be useful?

**Reporter**: [David Li](https://issues.apache.org/jira/browse/ARROW-17300) / @lidavidm

_{**Note**: *This issue was originally created as [ARROW-17300](https://issues.apache.org/jira/browse/ARROW-17300). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

先找出 Netty 與 Unsafe 記憶體後端的 Java 文件，然後檢查 Java 基準測試套件，尋找相關比較。記錄各個後端較適用的情況，並加入有用的基準測試參考；當文件清楚比較兩種選擇及其取捨時，即表示完成。

由索引模型根據 Issue 內容生成。