Vignette: Use LOO CV with validate_search = FALSE instead of K-fold CV

[fweber144]

This makes the vignette use cv_varsel() with LOO CV and validate_search = FALSE instead of K-fold CV. This provides a speed improvement when building the vignette and should reflect the typical projpred workflow better (running cv_varsel() with validate_search = FALSE first for rough results and only later running cv_varsel() with validate_search = TRUE). Of course, this causes the vignette to show an unvalidated search, but this is emphasized in the text, so I think that should be ok.

@avehtari: Do you want to take a look at this? You don't need to, but you contributed some helpful comments on the last vignette PR (#237). I added you as reviewer, but could remove you again.

[avehtari]

After running cv_varsel with validate_search = FALSE I would add something like

we directly plot with deltas = TRUE here (note that as the validate_search = FALSE this result is slightly optimistic, and the plot looks different when validate_search = TRUE is used):

close enough to the reference model's performance (this is also affected by validate_search = FALSE)

close enough to the reference model's performance (this is also affected by validate_search = FALSE)

same final submodel size (6) as by our manual decision (suggest_size() is also affected by validate_search = FALSE)

[avehtari]

Otherwise looks good