chore: finish the extendr.rs URL move (README, pkgdown)

[CGMossa]

TODO: replace this line with your own framing, in your own voice.

AI-written details

Summary

Finishes the extendr.github.io to extendr.rs move that #10 started on DESCRIPTION. Two commits, deliberately separate because they carry different risk:

• d39bac9 README (README.md:7,13 and the README.qmd:9,14 mirrors). Clears a real R CMD check --as-cran NOTE.
• 2d57150 pkgdown (_pkgdown.yml:1). Cosmetic on its own, but changes canonical-link and sitemap.xml output on the live site.

Drop either commit without unpicking the other if you only want one.

Why the README change matters

README.md is not in .Rbuildignore, so it ships in the tarball and CRAN attributes its links to tomledit. Both old URLs 301-redirect, and R CMD check --as-cran on current main reports:

Found the following (possibly) invalid URLs:
  URL: https://extendr.github.io/ (moved to https://extendr.rs/)
    From: README.md
    Status: 301
  URL: https://extendr.github.io/extendr/extendr_api/ (moved to https://extendr.rs/extendr/extendr_api/)
    From: README.md
    Status: 301

This is pre-existing on main, not a regression from #10.

README.md was hand-edited rather than re-rendered from README.qmd. A re-render would restamp the embedded example output dates (7fb9808 shows that happening: date = 2025-02-20 became 2025-03-03), which would bury a 2-line URL fix in unrelated churn.

Verification (local, R 4.6.0)

R CMD check --as-cran on the built tarball is down to Status: 2 NOTEs, with no invalid-URL findings at all. What remains is unrelated to this PR:

• CRAN incoming feasibility now contains only Version contains large components (0.1.1.9000), expected for a dev version.
• checking for hidden files and directories picks up a local .claude/ directory in my working tree. It does not exist in a fresh checkout, so CI will not see it.

urlchecker::url_check(".") goes from 2 findings to 0. Scope note: urlchecker reads DESCRIPTION, man/, README.md and NEWS.md, so that 0 validates the README.md lines specifically. The README.qmd edit is source-sync only (it is .Rbuildignored and never checked), and _pkgdown.yml is not in urlchecker's scope either. It parses cleanly to url: https://extendr.rs/tomledit/ and the pkgdown workflow job is the real check on it.

Noticed but deliberately not changed

The extendr badge label reads extendr-^0.7.1 while src/rust/Cargo.toml:13 pins extendr-api = "0.8.2". Only the badge's link target was touched here, not the version it asserts, since whether that should track Cargo.toml or advertise a supported floor is your call. Happy to bump it if you want.

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Drafted by Claude (claude-opus-5). Reviewed by the author.