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. :)

Start by reading the README example and the existing files in the examples directory. Identify the missing guidance around accessing components parsed from the AST, then add focused examples that make this usage and extension path clearer; done means the examples explain the relevant parser results beyond the current coverage.

Written by the indexing model from the issue text.