Update of the basics of DML article; new score for PLIV; adaption due to changed API of DoubleMLPLR & DoubleMLPLIV

[MalteKurz]

This PR contains:

• A major update of the basics of DML article. The graphics are now produced for the standardized estimates and compared to a normal density. Results for both the partialling out score and the IV-type score are shown. Additionally, the graphics have been prettified.
• Documentation for the newly added score IV-type for the PLIV model (see Python: Adapt nuisance est for IV-type score (PLR) & new score IV-type for PLIV doubleml-for-py#151 and R: Adapt nuisance est for IV-type score (PLR) & new score IV-type for PLIV doubleml-for-r#161)
• All articles, user guide etc have been updated in accordance with the changed API of DoubleMLPLR & DoubleMLPLIV (see Python: Adapt nuisance est for IV-type score (PLR) & new score IV-type for PLIV doubleml-for-py#151 and R: Adapt nuisance est for IV-type score (PLR) & new score IV-type for PLIV doubleml-for-r#161)

[MalteKurz]

• Build with released packages is expected to fail (updates in the pkg code needs to be released first)
• Build with dev packages is also expected to fail until Adapt nuisance est for IV-type score (PLR) & new score IV-type for PLIV doubleml-for-py#151 and Adapt nuisance est for IV-type score (PLR) & new score IV-type for PLIV doubleml-for-r#161 are merged
• However, with packages from branches the build can be triggered successfully, see https://github.com/DoubleML/doubleml-docs/actions/runs/2358365697

[PhilippBach]

Thanks @MalteKurz for preparing this PR which includes a lot of changes regarding the (names) of the nuisance learners as well as the example in the basics chapter. I only have a minor comment/suggestion for the plots in R there