Add dedicated vignettes for conditional correlations and contrast patterns

[Copilot]

This adds dedicated documentation for two pattern families that were previously covered only inside the general nuggets vignette. It also updates vignette navigation so the new material is discoverable from pkgdown, the README, the site index, and related articles.

• New vignettes

  • Added vignettes/conditional-correlations.Rmd focused on dig_correlations()
  • Added vignettes/contrast-patterns.Rmd covering:
    • dig_baseline_contrasts()
    • dig_complement_contrasts()
    • dig_paired_baseline_contrasts()

• Content structure

  • Split the overview material from vignettes/nuggets.Rmd into dedicated, task-oriented articles
  • Expanded the contrast-patterns material beyond the overview section with:
    • a reusable example dataset
    • parameter-oriented sections
    • examples for baseline, complement, and paired contrasts
    • links to related vignettes and function families
  • Built the conditional-correlations vignette from the existing overview plus R/dig_correlations.R documentation

• Navigation and discoverability

  • Added both vignettes to _pkgdown.yml
  • Added a vignette index section to both README.Rmd and index.Rmd
  • Added cross-links from existing vignettes where the new articles are natural follow-ons
  • Added direct links from the main nuggets vignette to the new detailed articles

• Example

  corr_basic <- dig_correlations(
      iris_corr,
      condition = where(is.logical),
      xvars = c(Sepal.Length, Sepal.Width, sepal_ratio),
      yvars = c(Petal.Length, Petal.Width, petal_ratio),
      min_length = 1,
      max_length = 1,
      min_support = 0.2
  )

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.90%. Comparing base (bf18cc5) to head (dc750f2).

Additional details and impacted files

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  Coverage   86.90%   86.90%           
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  Files         122      122           
  Lines        6392     6392           
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  Hits         5555     5555           
  Misses        837      837           

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