Add GitHub Actions CI/CD: R-CMD-check and pkgdown

[bailliem]

Summary

• Add R-CMD-check workflow with 3-config matrix (Ubuntu release/oldrel-1, macOS release)
• Add pkgdown workflow to build and deploy documentation to GitHub Pages
• Add .gitignore and .Rbuildignore for standard R package conventions
• Update _pkgdown.yml URL to point to GitHub Pages
• Add R-CMD-check status badge to README

Post-merge setup

After merging, enable GitHub Pages in repo settings:

• Source: Deploy from gh-pages branch, root (/)

Test plan

• R-CMD-check workflow triggers on push to main
• R-CMD-check passes on all 3 matrix configs
• pkgdown workflow builds documentation site
• pkgdown deploys to gh-pages branch
• Badge renders correctly in README
• Site accessible at https://openpharma.github.io/knockofftools/