Improve documentation

[jgardona]

Description

Docs is very poor, and what I understood about the cli is it only works with sql. Looked polars sql documentation but there are few contents also. Cant know what is the commands I can use while working with dataset, than I think some examples are necessary.

[stinodego]

It would be nice if we could set up a simple MkDocs and publish it with GitHub pages, similar to how it's done in the main repo.

We could include some more usage examples / installation instructions and include a full overview of supported functionality.

This is currently not a priority for me. If someone wants to pick this up, I would be happy to review a contribution.