added disclaimer for old documentation

Author: talbring

_layouts/docs.html

@@ -10,6 +10,13 @@
 
         <div class="col-md-8">
             <h1>{{ page.title }}</h1>
+
+            <b><font color="red">This page is only valid for SU2 version 6.2.0 and lower.</font></b>
+            <br>
+            <b><font color="red">The new documentation section can be found </font></b><a href="{{ "/docs_v7/home" | prepend: relative_url }}">here</a>.
+            <br>
+            <hr>
+
             <div id="markdown-content-container">{{ content }}</div>
             <p class="text-center">
               <br />