help: Update "Recent Conversations" to document collapsed view.

- Replaces special link for logged-in users with instructions so that
  users can learn how to access this view.

Fixes #27326.

help/include/go-to-recent-conversations.md

@@ -0,0 +1,4 @@
+1. Click on <i class="zulip-icon zulip-icon-clock"></i> **Recent conversations**
+   (or <i class="zulip-icon zulip-icon-clock"></i> if the **views**
+   section is collapsed) in the left sidebar,
+   or use the <kbd>T</kbd> keyboard shortcut.

help/include/recent-conversations.md

@@ -6,7 +6,7 @@ for catching up on messages sent while you were away.
 
 {tab|desktop-web}
 
-{relative|message|recent}
+{!go-to-recent-conversations.md!}
 
 1. The filters at the top help you quickly find relevant conversations.
    For example, select **Participated** to narrow to the conversations you

help/marking-messages-as-read.md

@@ -83,7 +83,7 @@ stream or topic as read**.
 
 {tab|via-recent-conversations}
 
-{relative|message|recent}
+{!go-to-recent-conversations.md!}
 
 1. Click on an unread messages counter in the **Topic** column to mark all
    messages in that topic as read.