Vignette for updated `ppc_loo_pit_ecdf` and `ppc_pit_ecdf` incl. correlated method

[florence-bockting]

Description

This Issue relates to PR#428 where we update the ppc_loo_pit_ecdf and ppc_pit_ecdf functions in agreement with the new work of Tesso & Vehtari (2026, see preprint).
The introduction of an additional method with different types of uniformity tests requires a Vignette that provides the user with some guiding examples and background information.

To reduce the scope of PR#428, this Issue aims at outsourcing the development of a corresponding Vignette to an additional PR.

[jgabry]

I agree, thanks!