Add IRC, Discord, SO links to "contribute" section at top of homepage

index.html

@@ -8,7 +8,7 @@
 
   <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
   <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
-  <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.4.2/css/all.css" integrity="sha384-/rXc/GQVaYpyDdyxK+ecHPVYJSN9bmVFBvjA/9eOB+pb3F2w2N6fc5qB9Ew5yIns" crossorigin="anonymous">
+  <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.15.4/css/all.css" crossorigin="anonymous">
 
   <link rel="alternate" href="https://github.com/GrammaticalFramework/gf-core/" title="GF GitHub repository">
 </head>
@@ -85,10 +85,27 @@
   <div class="col-sm-6 col-md-3 mb-4">
     <h3>Contribute</h3>
     <ul class="mb-2">
-      <li><a href="http://groups.google.com/group/gf-dev">Mailing List</a></li>
+      <li>
+        <a href="https://web.libera.chat/?channels=#gf">
+          <i class="fas fa-hashtag"></i>
+          IRC
+        </a>
+        /
+        <a href="https://discord.gg/EvfUsjzmaz">
+          <i class="fab fa-discord"></i>
+          Discord
+        </a>
+      </li>
+      <li>
+        <a href="https://stackoverflow.com/questions/tagged/gf">
+          <i class="fab fa-stack-overflow"></i>
+          Stack Overflow
+        </a>
+      </li>
+      <li><a href="https://groups.google.com/group/gf-dev">Mailing List</a></li>
       <li><a href="https://github.com/GrammaticalFramework/gf-core/issues">Issue Tracker</a></li>
-      <li><a href="doc/gf-people.html">Authors</a></li>
       <li><a href="//school.grammaticalframework.org/2020/">Summer School</a></li>
+      <li><a href="doc/gf-people.html">Authors</a></li>
     </ul>
     <a href="https://github.com/GrammaticalFramework/" class="btn btn-primary ml-3">
       <i class="fab fa-github mr-1"></i>
@@ -154,7 +171,7 @@ least one, it may help you to get a first idea of what GF is.
 <div class="row">
 
   <div class="col-md-6">
-    <h2>Applications & Availability</h2>
+    <h2>Applications & availability</h2>
     <p>
     GF can be used for building
     <a href="//cloud.grammaticalframework.org/translator/">translation systems</a>,