docs: Add documentation on AST and Cursor

I really like the latest additions to the decoding documentation.
However, I feel that the concepts of the AST and Cursor aren't really introduced/explained to the extent that the examples would make sense for someone who isn't already familiar with the concepts.

I jotted down a little something. But I'm also happy to make changes or see this changed by the maintainers.

Also, since the decoding.md page is getting pretty long, it might be worth changing some of the overall structure of the documentation, maybe with an additional level of page nesting.
That's of course not mine to decide, but I'd be happy to contribute to it, if I can.

zio / zio-json

docs: Add documentation on AST and Cursor #1098