Vignette for use of ppc*pit_ecdf() with new method argument

[florence-bockting]

Fixes #513

Description

This PR introduces a new vignette demonstrating the use of the ppc_pit_ecdf(), ppc_loo_pit_ecdf(), and ppc_pit_ecdf_grouped() functions. Specifically, it covers the new implementation introduced in #428, which added support for dependence-aware uniformity tests.

The vignette focuses on the newly introduced arguments and provides examples of how to interpret the correlation-aware ECDF plots.

TODO

• Merge Dependence aware tests for ppc*ecdf #428
• Create vignette as online-only article

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.79%. Comparing base (ad9a76f) to head (35af978).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #527   +/-   ##
=======================================
  Coverage   98.79%   98.79%           
=======================================
  Files          35       35           
  Lines        6582     6583    +1     
=======================================
+ Hits         6503     6504    +1     
  Misses         79       79           

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[jgabry]

@florence-bockting is this ready for review now that #428 was merged?

[florence-bockting]

@florence-bockting is this ready for review now that #428 was merged?

Thanks for pinging me. I checked the vignette again and made some final adjusts.
Good for review now. Thank you.