Deploy a snapshot of the specification to GitHub pages

This makes it easier to view the latest snapshot without having to download a
PDF from the releases page. The snapshots are deployed at the following URLs:
- [Unprivileged spec](https://riscv.github.io/riscv-isa-manual/snapshot/unprivileged/).
- [Privileged spec](https://riscv.github.io/riscv-isa-manual/snapshot/privileged/).

The HTML is deployed to a snapshot/ subdirectory to make it possible to also
deploy versioned releases in the future.

Author: arichardson

.github/workflows/isa-build.yml

@@ -129,3 +129,36 @@ jobs:
       env:
         GITHUB_TOKEN: ${{ secrets.GHTOKEN }}
 
+    # Create directory structure pages sites for privileged and unprivileged specs.
+    - name: Make GitHub pages directory
+      run: |
+        mkdir -p dist/snapshot/unprivileged dist/snapshot/privileged
+        cp build/riscv-unprivileged.html dist/snapshot/unprivileged/index.html
+        cp build/riscv-privileged.html dist/snapshot/privileged/index.html
+      if: github.event_name == 'workflow_dispatch' || (github.event_name == 'push' && github.ref == 'refs/heads/main')
+
+    - name: Upload pages artifact
+      uses: actions/upload-pages-artifact@v3
+      with:
+        path: dist
+      if: github.event_name == 'workflow_dispatch' || (github.event_name == 'push' && github.ref == 'refs/heads/main')
+
+  # Deploy HTML to Github pages.
+  deploy:
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+
+    needs: build
+
+    permissions:
+      pages: write
+      id-token: write
+
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+
+    runs-on: ubuntu-latest
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4