Add a banner to advertise Proof Assistants SE.

incl/footer.html

@@ -11,7 +11,10 @@
   <div id="sidebar">
     <div id="block-aggregator-feed-1" class="clear-block block block-aggregator">
       <h2 class="title">Recent news</h2>
-      <div class="content">
+      <div style="background-color: blanchedalmond; padding-left: 5px;" class="content">
+        <p>There is now a Stack Exchange Q&amp;A site dedicated to <a href="https://proofassistants.stackexchange.com">Proof Assistants</a>! Do not hesitate to post and answer Coq questions there (use the <a href="https://proofassistants.stackexchange.com/questions/tagged/coq">coq</a> tag).</p>
+      </div>
+      <div style="padding-left: 5px;" class="content">
         <p>Check
         out <a href="https://coq.discourse.group/c/announcements/8">Discourse</a>
         and <a href="https://twitter.com/CoqLang">Twitter</a> for