Add Pages workflow to build and deploy pkgdown on main

[Copilot]

The repository lacked a GitHub Pages deployment workflow, so pkgdown site publishing was not being triggered. This change introduces a dedicated Pages workflow that builds the site and deploys it via GitHub Actions.

• Workflow introduction

  • Adds .github/workflows/pages.yml as the Pages pipeline entrypoint.
  • Enables triggers on push to main and workflow_dispatch.

• Pages deployment contract

  • Sets required Pages permissions: contents: read, pages: write, id-token: write.
  • Adds workflow concurrency (group: "pages", cancel-in-progress: true) to serialize deployments.

• Build and publish flow

  • build-site job provisions R/Pandoc, installs website deps, and builds pkgdown output to docs.
  • Uploads docs as the Pages artifact.
  • deploy job publishes with actions/deploy-pages@v4 under the github-pages environment.

jobs:
  build-site:
    steps:
      - name: Build pkgdown site
        run: pkgdown::build_site_github_pages(new_process = FALSE, install = FALSE)
        shell: Rscript {0}
  deploy:
    needs: build-site
    steps:
      - uses: actions/deploy-pages@v4