docs: make citation guidance unmissable (netplot)

[gvegayon]

Makes citation guidance for netplot hard to miss, and corrects the citation metadata.

Motivation: the package has far more users than citations, and the usual reason is that people cannot find what to cite. Every change below is aimed at that gap.

inst/CITATION

• New file — citation("netplot") previously returned only auto-generated boilerplate.
• Added the CRAN DOI 10.32614/CRAN.package.netplot to the package entry (CRAN began minting these in 2024).
• The year is now resolved from Date/Publication → Date → current year, so the entry cannot render as (????).

Documentation

• README: a > [!NOTE] "How to cite" callout near the top. Both the source (.qmd/.Rmd) and the rendered README.md are updated; the block is static markdown, so no re-render was required.
• Vignettes (3): a short citation callout under the front matter (Quarto .callout-note in .qmd, a blockquote in .Rmd).
• ?netplot: a How to cite section, added to both the roxygen source and the generated .Rd so the two stay in sync (no roxygen re-run, keeping the diff small).
• Startup message: a one-line packageStartupMessage on attach pointing at citation("netplot"). It is suppressible in the normal way and does not fire on ::.

Verification

• utils::readCitationFile() parses the new file against the package DESCRIPTION, and the rendered entries were inspected.
• All R/*.R files still parse() and all man/*.Rd still pass tools::parse_Rd().
• Every DOI referenced here was checked to resolve; volumes, issues and pages were verified against CrossRef.

🤖 Generated with Claude Code

[github-actions]

Thank you for your contribution @gvegayon 🚀! The pkgdown site preview is ready for review 👉 Download here 👈!
_{(The artifact expires on 2026-11-04T07:02:14Z. You can re-generate it by re-running the workflow here.)}