added link to demo server

layouts/index.html

@@ -6,6 +6,8 @@
     </div>
     <div class="col-lg-9 col-xl-8 text-center">
       <p class="lead">{{ .Params.lead | safeHTML }}</p>
+      <a class="btn btn-primary btn-lg px-4 mb-2" href="https://demo.mcaptcha.org/" role="button">Try mCaptcha</a>
+      &nbsp &nbsp &nbsp
       <a class="btn btn-primary btn-lg px-4 mb-2" href="{{ "docs/prologue/introduction/" | absURL }}" role="button">Get started</a>
       <p class="meta">Libre software <a href="https://github.com/mCaptcha">GitHub</a></p>
     </div>