Remix some community channels (move to libera.chat) (#1955)

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

content/community.html

@@ -16,8 +16,15 @@ <h1>Community</h1>
     </p>
     <p>
       <strong>IRC:</strong>
-      <a href="https://webchat.freenode.net/?channels=#prometheus"><code>#prometheus</code></a> on <a href="http://freenode.net/">irc.freenode.net</a>
-      (for the easiest start, <a href="https://app.element.io/#/room/#prometheus:matrix.org">join via Element</a>)
+      <code>#prometheus</code> on <a href="https://libera.chat/">irc.libera.chat</a>
+    </p>
+    <p>
+      <strong>Matrix:</strong>
+      <a href="https://app.element.io/#/room/#prometheus:matrix.org"><code>#prometheus:matrix.org</code></a>. Note: we are working on bridging matrix and libera.chat together.
+    </p>
+    <p>
+      <strong>Community-maintained slack channel:</strong>
+      <code>#prometheus</code> on CNCF <a href="https://slack.cncf.io/">slack</a>.
     </p>
     <p>
       <strong>User mailing lists:</strong>
@@ -84,7 +91,7 @@ <h1>Contributing</h1>
     </p>
     <p>
       <strong>IRC:</strong>
-      <a href="https://webchat.freenode.net/?channels=#prometheus-dev"><code>#prometheus-dev</code></a> on <a href="http://freenode.net/">irc.freenode.net</a>
+      <code>#prometheus-dev</code> on <a href="https://libera.chat/">irc.libera.chat</a>
     </p>
     <p>
       <strong>Development mailing list:</strong>