Merging documents of `pl_ClassName` and `ClassName_class`

[eitsupi]

We currently have Rd files for functions that creates an instance of a class (e.g. pl_DataFrame)and an Rd file that describes the class itself (e.g. DataFrame_class), but I think it would be easier to understand if these were displayed on one page. https://github.com/pola-rs/r-polars/blob/e457053db4c4e4547975bca326e2a1271e193257/man/DataFrame_class.Rd https://github.com/pola-rs/r-polars/blob/e457053db4c4e4547975bca326e2a1271e193257/man/pl_DataFrame.Rd

I don't think there is any problem in merging the Rd files, but since the website currently separates functions by namespace, it may become difficult to understand if page pl$DataFreame() and method pages <DataFrame>$func() are classified into different subcategories. Perhaps we should do something like leave DataFrame class as an alias to pl$DataFrame() on the website?