Update documentation to changes `paradox` (breaking)

[PhilippBach]

Description of the issue or change proposal

Update the docu w.r.t. breaking changes of paradox, see DoubleML/doubleml-for-r#194

Comments, context or references

Related PR & diff: DoubleML/doubleml-for-r#194 , https://github.com/mlr-org/mlr3pipelines/pull/752/files#diff-bccd0d30e65d0fece49968c6735dd5e0c7974e8f8b4589e1168cf421555729aa

Affected sites: https://docs.doubleml.org/stable/guide/learners.html

TODO:

• Double check examples gallery

[PhilippBach]

done in #172