help: Separate quote-and-reply into its own article.

templates/zerver/help/include/sidebar_index.md

@@ -32,10 +32,11 @@
 * [Share and upload files](/help/share-and-upload-files)
 * [Enable Enter to send](/help/enable-enter-to-send)
 * [Verify a message was sent](/help/verify-your-message-was-successfully-sent)
+* [Quote and reply](/help/quote-and-reply)
 * [@-mention a user](/help/at-mention-a-user)
+* [Notify everyone on a stream](/help/notify-everyone-on-a-stream)
 * [Edit or delete a message](/help/edit-or-delete-a-message)
 * [Message a stream by email](/help/message-a-stream-by-email)
-* [Notify everyone on a stream](/help/notify-everyone-on-a-stream)
 
 ## Reading messages
 * [Navigation and unread counts](/help/unread-counter-and-pointer)

templates/zerver/help/quote-and-reply.md

@@ -0,0 +1,20 @@
+# Quote and reply
+
+You can quote a previous message in your reply.
+
+Note that an [@-mention](/help/at-mention-a-user) inside quoted text will
+not mention the user, so feel free to quote and reply without worrying about
+spurious mentions.
+
+### Quote and reply
+
+{start_tabs}
+
+{!message-actions-menu.md!}
+
+1. Click **Quote and reply**.
+
+{end_tabs}
+
+You can also use the keyboard shortcut `>` to quote the message under the
+blue box.