Add support for polars.Enum

[shenrun]

The polars.Enum does not work in data model definition.

import pandera.polars as pa
from pandera.typing import Series
class Foo(pa.DataFrameModel):
#    emum: pl.Enum(['a','b']) 
    emum: Series[pl.Enum] = pa.Field(dtype_kwargs={"categories":('a','b')})

[ksolarski]

This now has been done in #1798. However, I believe the documentation on the website hasn't been updated as it doesn't list Enum data type under polars engine.