Add GitHub Actions workflow to deploy R documentation

This workflow builds and deploys the R documentation site from
the graphing-library-docs repository to this repo's GitHub Pages.

The workflow:
- Runs weekly (Sundays) to pick up docs changes
- Can be triggered manually via workflow_dispatch
- Can be triggered via repository_dispatch from the docs repo

The docs source lives at cpsievert/graphing-library-docs and includes
documentation for both plotly.R and ggplotly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cpsievert

.github/workflows/docs.yml

@@ -0,0 +1,79 @@
+# Add this file to plotly/plotly.R at .github/workflows/docs.yml
+#
+# This workflow builds and deploys the R documentation site from
+# the graphing-library-docs repository to this repo's GitHub Pages.
+
+name: Deploy R Documentation
+
+on:
+  # Manual trigger
+  workflow_dispatch:
+
+  # Run weekly to pick up any docs changes
+  schedule:
+    - cron: '0 0 * * 0'  # Every Sunday at midnight UTC
+
+  # Optional: trigger from docs repo via repository_dispatch
+  repository_dispatch:
+    types: [docs-updated]
+
+# Sets permissions for GitHub Pages deployment
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+# Allow only one concurrent deployment
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+
+env:
+  # Change this if the docs repo moves to a different location
+  DOCS_REPO: cpsievert/graphing-library-docs
+  DOCS_BRANCH: master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout docs repository
+        uses: actions/checkout@v4
+        with:
+          repository: ${{ env.DOCS_REPO }}
+          ref: ${{ env.DOCS_BRANCH }}
+
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.1'
+          bundler-cache: true
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Validate front-matter
+        run: python front-matter-ci.py _posts
+
+      - name: Build site
+        run: bundle exec jekyll build
+        env:
+          JEKYLL_ENV: production
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: _site
+
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4