Add polars-df to data_analysis/structures category

[andynu]

Polars is a data frame library
https://rubygems.org/gems/polars-df

Thanks for contributing to the Ruby Toolbox catalog! <3

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• [?] Make sure the CI build passes, we validate your proposed changes in the test suite :)
  We will see if it passes, hope so.

[colszowka]

Thank you @andynu 👍