consider provide more examples

Thanks for the project! I am just playing with the parser to get used to it. However, I could not figure out how we get to the components that were parsed from the AST parser. I followed an example as described in [here](https://github.com/ballista-compute/sqlparser-rs/#example). Since only a few examples are [given](https://github.com/ballista-compute/sqlparser-rs/tree/main/examples), I think it is a bit off to understand and extend this library by following only those examples. It's better to add more examples. :)

先閱讀 README 範例和 examples 目錄中的現有檔案。找出關於存取從 AST 解析出的元件之相關指引缺口，然後加入聚焦的範例，讓這種使用方式和擴充路徑更加清楚；完成的標準是範例能解釋超出目前涵蓋範圍的相關 parser 結果。

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