chore(pkgdown): wire GH Actions deploy to gh-pages

[VincentGuyader]

Summary

Sets up pkgdown site generation and automated deployment to GitHub Pages via GitHub Actions.

Changes

• _pkgdown.yml (new): site URL https://vincentguyader.github.io/shiny2docker/, Bootstrap 5 template.
• .github/workflows/pkgdown.yaml (new): standard r-lib/actions setup; runs on push to main, on PRs (build only), on release, and on manual dispatch. Uses pkgdown::build_site_github_pages() then JamesIves/github-pages-deploy-action@v4.5.0 to push to the gh-pages branch (deploy step skipped on PRs).
• .Rbuildignore: exclude _pkgdown.yml, docs/, pkgdown/ from the package tarball.
• .gitignore: exclude /docs/ (site is deployed via Action, not committed to main).

After merge

Enable GitHub Pages in repo Settings -> Pages:

• Source: Deploy from a branch
• Branch: gh-pages
• Folder: / (root)

The first push to main after merge triggers the workflow, creates the gh-pages branch, and publishes the site.

Local validation

• pkgdown::build_site_github_pages(new_process = FALSE, install = FALSE) -> clean build, no URL warning, 12 pages + .nojekyll.
• CI on this PR will exercise the build step (deploy step is skipped on PRs).

Test plan

• Site builds locally with the same command the workflow uses.
• After merge: first run on main creates gh-pages branch.
• After GitHub Pages enablement: site reachable at the URL declared in _pkgdown.yml.