Add r2u documentation for fast binary package installation

[Copilot]

Documents r2u, which provides pre-compiled binary packages for all CRAN packages on Ubuntu, reducing installation times from minutes to seconds.

Changes

• Continuous Integration chapter: New section explaining r2u for GitHub Actions workflows

  • Benefits: faster CI builds, automatic dependency resolution, reduced Actions minutes
  • Example GitHub Actions integration using eddelbuettel/github-actions/r2u-setup@master
  • When to use: Ubuntu runners with many package dependencies

• Reproducible Environments chapter: New section positioning r2u relative to renv

  • Comparison guidance: r2u for speed on Ubuntu, renv for cross-platform reproducibility, or both together
  • Supported Ubuntu versions: 24.04 noble, 22.04 jammy, 20.04 focal (archived)
  • Docker containers: rocker/r2u for containerized workflows

Both sections include links to official r2u documentation for setup instructions.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>summarize and link to https://eddelbuettel.github.io/r2u/</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes summarize and link to https://eddelbuettel.github.io/r2u/ #187

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[github-actions]

PR Preview Action v1.8.1-1-g6f3a89d
🚀 View preview at https://UCD-SERG.github.io/lab-manual/pr-preview/pr-188/
Built to branch gh-pages at 2026-02-02 03:46 UTC. Preview will be ready when the GitHub Pages deployment is complete.