CI: Upload documentation HTML ZIP archive and PDF file as release assets (#3814)

Author: seisman

.github/workflows/ci_docs.yml

@@ -151,6 +151,12 @@ jobs:
         run: cp -v doc/_build/pygmt-docs.zip doc/_build/pygmt-docs.pdf doc/_build/html/
         if: github.event_name == 'push' && matrix.os == 'ubuntu-latest'
 
+      - name: Upload the HTML ZIP archive and PDF as release assets
+        run: gh release upload ${{ github.ref_name }} doc/_build/pygmt-docs.zip doc/_build/pygmt-docs.pdf
+        if: github.event_name == 'release' && matrix.os == 'ubuntu-latest'
+        env:
+          GH_TOKEN: ${{ github.token }}
+
       - name: Checkout the gh-pages branch
         uses: actions/[email protected]
         with: