Misssing methods for `struct` Series: `fields`, `unnest`, `schema`

[Yunuuuu]

https://github.com/pola-rs/polars/blob/main/py-polars/polars/series/struct.py

Sorry, I don't know how to transform this rust code into r-polars. https://github.com/pola-rs/polars/blob/main/py-polars/src/series/mod.rs#L77

It easily to use $to_frame and $unnest to implement $unnest() method in R, but I see python call rust method, I don't know if it's good idea to implement in R side only.

struct_unnest = function() self$to_frame()$unnest()

[eitsupi]

These seem fairly easy to implement, but updating the website to display the Series subnamespace in the sidebar is quite cumbersome (Related to #686).

[Yunuuuu]

It would be great if the website can display the full series method (some methods from expr object) like python reference. when I learn r-polars the first time from the r-polars website, I consider $filter and $slice method (other methods displayed under Expressions) cannot be used for Series.

[eitsupi]

All that is needed here is for someone to write the documentation. However, I don't know if it is worth adding more Rd files to the 600+ that already exist.