v1.8.1 — corrected README examples, new website articles, lint

Pre-release from main (merge commit 3c5b134); no change to returned values or the API since 1.8.0. CRAN still serves 1.6.1; install with pak::pak("Data-Wise/rmediation@v1.8.1").

No change to returned values or to the API.

Documentation

• The README examples for medci() and ci() on a ProductNormal3 now show the real output of those calls (they
  showed intervals that the functions do not return), and the Monte Carlo calculator of the online apps is linked.
• The SEMFit mbco() route is no longer described as experimental: medfit dropped that label in 0.11.0.
• The pkgdown site gains a Quick Reference, a Cookbook (including serial mediation through medfit, the SEMFit route
  and omitted-confounder sensitivity) and Technical Notes (the exact CDF at |rho| = 1, point masses, the densities).

Internal

• Added .lintr (120 columns) and wrapped 61 over-long or mis-indented lines in R sources and tests; the parsed code
  is unchanged. Rewrapped roxygen lines regenerate seven help pages with the same content.