ci: add docs publish workflow for GitHub Pages

[pratyush618]

Summary

• Adds a GitHub Actions workflow to build and deploy the Docusaurus documentation site to GitHub Pages
• Triggers on pushes to main that modify docs/** or the workflow itself, plus manual dispatch
• Uses actions/upload-pages-artifact@v3 and actions/deploy-pages@v4 for deployment
• Concurrency group prevents overlapping deployments

Setup required after merge

Enable GitHub Pages in repo settings: Settings > Pages > Source > GitHub Actions

Test plan

• Verify workflow triggers on docs changes to main
• Verify Docusaurus site builds successfully in CI
• Verify deployment to GitHub Pages after enabling the source