help: Add mobile documentation for "Follow a topic".

Adds instructions for the Flutter mobile app in the "Follow a topic"
article.

Part of #34748.

help/follow-a-topic.md

@@ -33,6 +33,12 @@ You can use followed topics for a variety of workflows:
 
 {!configure-topic-notifications-desktop-web.md!}
 
+{tab|mobile}
+
+{!topic-long-press-menu.md!}
+
+1. Tap **Follow topic** or **Unfollow topic**.
+
 {end_tabs}
 
 ## Catch up on followed topics
@@ -112,6 +118,19 @@ messages just in followed topics, without counting other channel messages.
 
 {!manage-configured-topics-desktop-web.md!}
 
+{tab|mobile}
+
+1. Tap the **Channels** (<i class="zulip-icon zulip-icon-hash mobile-help"></i>)
+   tab at the bottom of the app.
+
+1. Select a channel containing topics you want to configure.
+
+1. Tap **topics** in the upper right corner of the app.
+
+{!topic-long-press-menu.md!}
+
+1. Tap **Mute topic**, **Unmute topic**, or **Follow topic**.
+
 {end_tabs}
 
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