docs: Add Netlify badge if on Netlify

Author: bep

docs/layouts/partials/footer.html

@@ -3,6 +3,11 @@
 			  <p>Last revision: {{ .Lastmod.Format "January 2, 2006" }}{{ if .IsPage }}{{ with .GitInfo }} | <a href="https://github.com/spf13/hugo/commit/{{ .Hash }}">{{ .Subject }} ({{ .AbbreviatedHash }})</a>{{end }}{{ end }}
                           <span style="float: right;">Hugo v{{ .Site.Params.release }} documentation</span>
                         </p>
+                        {{ with  getenv "REPOSITORY_URL" -}}
+                        <a href="https://www.netlify.com" style="float: right; padding-right: 20px;">
+                        <img src="https://www.netlify.com/img/global/badges/netlify-color-bg.svg"/>
+                        </a>
+                        {{- end }}
                         </div>
                     </div>