include docs snapshot with pull request build so that PRs can preview it easily

.github/workflows/build-on-pull-request.yml

@@ -43,6 +43,11 @@ jobs:
       - name: Build the document files.
         shell: bash
         run: make -j2 doc
+      - name: Upload a documents snapshot.
+        uses: actions/upload-artifact@v3
+        with:
+          name: docs
+          path: ./html
       - name: Build 64-bit Windows versions of the tools.
         run: |
           make -C src clean