Add more usage examples to the documentation (e.g., parsing enums)

[leo-olivier]

Just a small suggestion: it would be great to have more examples in the documentation. The existing CSV example is helpful, but a limited. For instance, it doesn’t show how to parse into enums or handle more complex grammar structures. Adding a few additional examples would really help newcomers understand how to use pest_ast.

Anyway, awesome work on this crate :)