help: Add mobile instructions in "Starting a new topic".

- Adds mobile instructions block.
- Adds two more related articles.

help/include/how-to-start-a-new-topic.md

@@ -15,6 +15,12 @@
     You can change the destination stream for your message using the dropdown
     in the top left of the compose box.
 
+!!! warn ""
+
+    In Zulip, you can compose a message to a different place than the one you
+    are viewing. In this situation, the message feed will fade to indicate
+    what's going on.
+
 {tab|not-stream}
 
 1. Click the **New topic** button at the bottom of the app, or
@@ -28,10 +34,27 @@
 
 {!compose-and-send-message.md!}
 
-{end_tabs}
-
 !!! warn ""
 
     In Zulip, you can compose a message to a different place than the one you
     are viewing. In this situation, the message feed will fade to indicate
     what's going on.
+
+{tab|mobile}
+
+1. Tap the **Streams**
+   (<img src="/static/images/help/mobile-hash-icon.svg" alt="hash" class="mobile-icon"/>)
+   tab at the bottom of the app.
+
+1. Tap on the desired stream.
+
+1. Tap the compose box at the bottom of the app.
+
+1. Enter a topic name. Auto-complete will provide suggestions for previously
+   used topics.
+
+1. Compose your message, and tap the **send**
+   (<img src="/static/images/help/mobile-send-circle-icon.svg" alt="send" class="mobile-icon"/>)
+   button in the bottom right corner of the app.
+
+{end_tabs}

help/starting-a-new-topic.md

@@ -14,3 +14,5 @@
 * [Starting a new direct message](/help/starting-a-new-direct-message)
 * [Replying to messages](/help/replying-to-messages)
 * [Messaging tips & tricks](/help/messaging-tips)
+* [Rename a topic](/help/rename-a-topic)
+* [Require topics in stream messages](/help/require-topics)