Add GitHub Actions workflow for docs deployment to GitHub Pages

[kmcginnes]

Parent PRD

• Part of Migrate documentation to Starlight (Astro) #1754

What to build

Add a GitHub Actions workflow that builds the Starlight docs site and deploys it to GitHub Pages automatically when changes are pushed to main.

End-to-end, this means:

1. Create .github/workflows/deploy_docs.yml with:
   • Trigger on push to main with path filter: packages/docs/**
   • Manual workflow_dispatch trigger for ad-hoc deploys
   • Concurrency group to cancel in-progress deploys
   • Build job: checkout, pnpm setup, install, pnpm docs:build
   • Deploy job: upload packages/docs/dist/ as pages artifact, deploy via actions/deploy-pages
   • Appropriate permissions (pages: write, id-token: write, contents: read)
2. Enable GitHub Pages in repo settings with source set to "GitHub Actions" (manual step, document in the PR)
3. Verify the workflow runs successfully and the site is accessible at https://aws.github.io/graph-explorer/

Acceptance criteria

• .github/workflows/deploy_docs.yml exists and is valid
• Workflow triggers on push to main only when packages/docs/** files change
• Workflow triggers on manual workflow_dispatch
• Workflow builds the docs site and deploys to GitHub Pages
• Site is accessible at https://aws.github.io/graph-explorer/ after first deploy
• Concurrent pushes cancel in-progress deployments

Blocked by

• Blocked by Scaffold Starlight project with Tailwind, configure for GitHub Pages, and integrate with monorepo #1755

User stories addressed

• User story 6 (auto-deploy on merge)
• User story 8 (canonical documentation URL)