Add CJ9 page link to side bar

Technically it's an ongoing jam, not a previous jam. But I think adding this link will help with discoverability if you're already on the CJ Info page. Can be updated once a theme is announced

Author: minalike

pydis_site/templates/events/sidebar/code-jams/previous-code-jams.html

@@ -1,6 +1,7 @@
 <div class="box">
     <p class="menu-label">Previous Code Jams</p>
     <ul class="menu-list">
+        <li><a class="has-text-link" href="{% url "events:page" path="code-jams/9" %}">Code Jam 9: (Theme to be announced)</a></li>
         <li><a class="has-text-link" href="{% url "events:page" path="code-jams/8" %}">Code Jam 8: Think Inside the Box</a></li>
         <li><a class="has-text-link" href="{% url "events:page" path="code-jams/7" %}">Code Jam 7: Early Internet</a></li>
         <li><a class="has-text-link" href="{% url "events:page" path="code-jams/6" %}">Code Jam 6: Ancient Technology</a></li>