user docs: Add delete-a-topic.

templates/zerver/help/delete-a-topic.md

@@ -0,0 +1,24 @@
+# Delete a topic
+
+{!admin-only.md!}
+
+We generally recommend against deleting topics, but there are a few
+situations in which it can be useful:
+
+* Clearing out test messages after setting up an organization.
+
+* Clearing out messages from an overly enthusiastic bot.
+
+* Managing abuse.
+
+In most other cases, [renaming a topic](/help/rename-a-topic) is often a
+better idea, or just leaving the topic as is. Deleting a topic can confuse
+users who come to the topic later via an email notification.
+
+1. Hover over the topic in the **left sidebar**.
+
+1. Click the down chevron (<i class="fa fa-chevron-down"></i>).
+
+1. Click **Delete all messages in `<topic>`**.
+
+1. Click **Delete messages**.

templates/zerver/help/getting-your-organization-started-with-zulip.md

@@ -106,6 +106,7 @@ expert teaching other users how to use Zulip.
 - [Add custom emoji](/help/add-custom-emoji), at the very least for
   your organization's logo.
 - Link to your Zulip instance with a [nice badge](/help/join-zulip-chat-badge).
+- [Delete](/help/delete-a-topic) embarrassing test messages before sending invites.
 - Send feedback to the Zulip development community!  We love hearing
   about problems (however minor) and feature ideas that could make
   Zulip even better.

templates/zerver/help/include/sidebar_index.md

@@ -67,6 +67,7 @@
 * [Message a stream by email](/help/message-a-stream-by-email)
 * [Edit the topic of a message](/help/change-the-topic-of-a-message)
 * [Rename a topic](/help/rename-a-topic)
+* [Delete a topic](/help/delete-a-topic)
 
 ## Notifications
 * [Stream notifications](/help/stream-notifications)