Fix “nightly warning” (#16227)

Somehow, part of the work done in #15156 has been reverted. This PR
fixes it.

Closes #15115

Author: julienrf

docs/_layouts/static-site-main.html

@@ -4,7 +4,7 @@
 
 <div class="site-container">
   <div id="site-header"></div>
-  {% if page.movedTo %}
+  {% if page.nightlyOf %}
   <aside class="warning">
     This is a nightly documentation. The content of this page may not be
     consistent with the current stable version of language. Click