doc: Fix messsage for main branch (#283)

docs/overrides/main.html

@@ -1,8 +1,17 @@
 {% extends "base.html" %}
 
 {% block outdated %}
-  You're not viewing the latest version.
-  <a href="{{ '../' ~ base_url }}">
-    <strong>Click here to go to latest.</strong>
-  </a>
+  <div id="current-version"> </div>
+  <script>
+          let message = "You're not viewing the latest version."
+          // There is probably a nicer method retrieving the value of the
+          // version element but it is loaded after the outdated block
+          let current = window.location.pathname.split("/")[1]
+          if (current == "main") {
+                message = "This is the documentation from the 'main' branch which is not a stable release."
+          }
+          message = message + '<a href="{{ "../" ~ base_url }}"> <strong>Click here to go to latest stable release.</strong> </a>'
+          document.getElementById("current-version").innerHTML = message;
+   </script>
+  {{app}}
 {% endblock %}