docs: Remove remaining mentions of Matrix-IRC bridge

This is a fixup of
#1011 which did
not entirely remove all mentions

Also make the low participation in the IRC channel clearer

docs/community.rst

@@ -11,10 +11,9 @@ Getting in contact
 The main place of interaction for the Isso community is the
 `GitHub issue tracker <https://github.com/isso-comments/isso/issues>`_.
 
-A few people - including the lead developers and maintainers - frequent the
-``#isso`` channel, which you can join via
-`Matrix <https://matrix.to/#/#isso:libera.chat">`_ or via IRC on
-`Libera.Chat <https://libera.chat/>`_
+A few people - including some of the lead developers and maintainers -
+sometimes participate in the ``#isso`` channel, which you can join via IRC on
+`Libera.Chat <https://libera.chat/>`_.
 
 You may also ask questions or suggest improvements on
 `GitHub Discussions <https://github.com/isso-comments/isso/discussions>`_.

docs/index.html

@@ -46,8 +46,7 @@ <h2>Links</h2>
     </ul>
     <h2>Help</h2>
     <p>
-        Join <code>#isso</code> via <a href="https://matrix.to/#/#isso:libera.chat">Matrix</a>
-        or via IRC on <a href="https://libera.chat/">Libera.Chat</a>,
+        Join <code>#isso</code> via IRC on <a href="https://libera.chat/">Libera.Chat</a>,
         or ask a question on <a href="https://github.com/isso-comments/isso/discussions">GitHub&nbsp;Discussions</a>.
     </p>
     <h2>Contribute</h2>