pvstackr 0.2.1

This release changes the documentation only. The R code, the computations and
the printed output are the same as in 0.2.0.

• The help pages, the ten articles and the README are rewritten in plain
  language and checked against the code, the companion methods preprint and the
  cited literature.

• Statements that did not match the package are corrected:

  • pvstackr fits a single-level linear regression weighted by the survey
    weights and reports its fixed effects. A formula with a random-effect term
    stops pv_brr_target(), stack_direct and stack_psis with an error. The
    earlier articles presented a two-level model with a school variance.
  • The likelihood of each plausible value is survey-weighted, and each row of
    the stacked data has as its weight the final weight divided by its mean and
    by the number of plausible values. The earlier text called the likelihood
    unweighted and gave the row weight as one over the number of plausible
    values.
  • pvstackr runs no part of Pareto smoothed importance sampling: stack_psis
    takes importance weights and Pareto k-hat values computed elsewhere. The
    thresholds of Vehtari et al. (2017, 2024) are now stated apart from
    pvstackr's single threshold, psis_k_threshold.
  • The companion methods paper is cited as its Zenodo preprint
    (https://doi.org/10.5281/zenodo.22407935). Its stacked fixed-effect point
    identity is Theorem 4.1, with conditions (R1)–(R5). Coverage results that
    the preprint does not report are no longer attributed to it; the interval
    labels (interval_role, coverage_claim_allowed) are pvstackr's
    reporting rule.
  • The covariance that pv_brr_target() computes from the replicate weights
    is called the BRR–Fay replicate covariance, not a sandwich estimator.

• The interval label coverage_claim_allowed is described as what it is:
  a record of how an interval was built, by pvstackr's reporting rule. It does
  not certify the coverage of the interval.

• The package title and description are rewritten in plain words.

• The articles have new titles; their file names and web addresses are
  unchanged. The website has a "Get started" link to the first article.

• The tests that check phrases in the documentation follow the new wording and
  check the same facts.