docs(icons): Add discord link to the header

Author: jcs090218

docs/data/menu/extra.yaml

@@ -4,3 +4,7 @@ header:
     ref: https://github.com/emacs-eask/cli
     icon: gdoc_github
     external: true
+  - name: Discord
+    ref: https://discord.com/invite/E9zzjWGfFD
+    icon: gdoc_dicord
+    external: true

docs/static/custom.css

@@ -22,3 +22,12 @@
 .gdoc-footer {
   background: #44879A;
 }
+
+/* Icons */
+
+.gdoc_dicord {
+  background: url('./icons/discord-mark-white.svg');
+  background-repeat: no-repeat;
+  margin-top: 5px;
+  background-size: 90%;
+}