Add a section for selection-induced bias in model comparison

We should add a section in the model comparison chapter (probably chapter 7?) outlining the order statistic diagnostic in `commpare()` related to https://github.com/arviz-devs/arviz-stats/pull/237 with an example of thw warning that is surfaced.

Locate the model comparison chapter, likely chapter 7, and inspect the order statistic diagnostic in `commpare()` alongside arviz-stats PR 237. Add a section explaining selection-induced bias and include an example of the warning that is surfaced; the chapter should clearly describe the diagnostic and warning behavior.

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