Add extra script to the docs created in GitHub Actions workflow

This was done by the cron job running in docs.wxwidgets.org before, so
keep doing it here.

.github/workflows/docs_update.yml

@@ -20,6 +20,12 @@ jobs:
         run: |
           sudo apt-get -q -o=Dpkg::Use-Pty=0 -y install doxygen graphviz
 
+      - name: Customize for online docs
+        working-directory: docs/doxygen
+        run: |
+          sed -i'' -e "s@<!--EXTRA FOOTER SCRIPT-->@<script>(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)})(window,document,'script','//www.google-analytics.com/analytics.js','ga');ga('create', 'UA-794025-2', 'auto');ga('send', 'pageview');</script>@" \
+            custom_footer.html
+
       - name: Generate documentation
         working-directory: docs/doxygen
         run: |