Dataframes: Add section about the Polars Dataframe library

[amotl]

About

Accompanying other patches, this one adds a section about the Polars Dataframe library to the documentation.

• https://pola.rs/
• https://docs.pola.rs/

Preview

https://crate-clients-tools--65.org.readthedocs.build/en/65/connect/df.html#polars

References

• https://github.com/crate/cratedb-examples/issues/228
• https://github.com/crate/cratedb-examples/pull/229

/cc @hlcianfagna