[CI] Allow empty git commit when deploying docs to gh-pages (ukri-exc…

…alibur#309)

I believe with mkdocs
[v1.6.0](https://github.com/mkdocs/mkdocs/releases/tag/1.6.0) we may have to
allow empty commits, because the compressed sitemap shouldn't change anymore on
every rebuild.

.github/workflows/docs.yml

@@ -112,7 +112,7 @@ jobs:
           fi
           cp -fr ../site/* .
           git add .
-          git commit -m "${COMMIT_MESSAGE}"
+          git commit --allow-empty -m "${COMMIT_MESSAGE}"
           git push origin gh-pages
 
       - name: Create custom status for pull requests