Fix documentation link

Author: darsnack

_layout/navbar.html

@@ -13,7 +13,7 @@
         <a class="nav-link" href="/getting_started/">Getting Started</a>
       </li>
       <li class="nav-item">
-        <a class="nav-link" href="{{website_url}}/Flux.jl/" target="_blank">Docs</a>
+        <a class="nav-link" href="https://fluxml.ai/Flux.jl/" target="_blank">Docs</a>
       </li>
       <li class="nav-item">
         <a class="nav-link" href="/blog/">Blog</a>