Update documentation with not in place transforms and update DatasetDict
In #1883 were added the not in-place transforms `flatten`, `remove_columns`, `rename_column` and `cast`.

I added them to the documentation and added a paragraph on how to use them

You can preview the documentation [here](https://28862-250213286-gh.circle-artifacts.com/0/docs/_build/html/processing.html#renaming-removing-casting-and-flattening-columns)

I also added these methods to the DatasetDict class.