help: Rename left-sidebar-topics > left-sidebar-conversations.

- Renames Markdown include and references for consistency.

Fixes part of #27191.

help/finding-a-topic-to-read.md

@@ -16,7 +16,7 @@
 
 ## From the left sidebar
 
-{!left-sidebar-topics.md!}
+{!left-sidebar-conversations.md!}
 
 ## Related articles
 

help/getting-started-with-zulip.md

@@ -36,7 +36,7 @@ it, you'll never want to use a different team chat app!
 
 #### From the left sidebar
 
-{!left-sidebar-topics.md!}
+{!left-sidebar-conversations.md!}
 
 ### Reading conversations
 

help/reading-strategies.md

@@ -28,7 +28,7 @@ in Zulip.
 
 #### From the left sidebar
 
-{!left-sidebar-topics.md!}
+{!left-sidebar-conversations.md!}
 
 ### Reading conversations