user docs: Add macro for opening the compose box.

docs/subsystems/user-docs.md

@@ -264,6 +264,10 @@ Save changes after modifying organization settings.
 
 Navigate to the stream actions menu from the left sidebar.
 
+### **Start composing** `{!start-composing.md!}` macro
+
+Open the compose box.
+
 ### Tips and warnings
 
 A **tip** is any suggestion for the user that is not part of the main set of

templates/zerver/help/add-emoji.md

@@ -5,7 +5,7 @@ shows you how to send messages containing emoji by using Zulip’s built-in emoj
 menu, pasting Unicode, or manually inputting the emoji code.
 
 ## Using the emoji menu
-1. Compose a [new message or reply](/help/reply-to-a-message).
+{!start-composing.md!}
 2. Click the smiley face (<i class="icon-vector-smile"></i>) icon in the
 bottom left corner of the compose box.
 3. Upon clicking on the smiley face (<i class="icon-vector-smile"></i>)
@@ -16,7 +16,7 @@ the emoji menu to include it in your message.
 Zulip fully supports Unicode emoji. To use Unicode emoji in your message:
 
 1. Copy any emoji you would like to include onto your clipboard, such as 😀.
-2. Compose a [new message or reply](/help/reply-to-a-message).
+{!start-composing.md!}
 3. Paste the emoji into the **Compose your message here...** field of the
 compose box.
 
@@ -26,7 +26,7 @@ compose box.
     however, Zulip emoji may look different than the ones listed.
 
 ## Using emoji codes
-1. Compose a [new message or reply](/help/reply-to-a-message).
+{!start-composing.md!}
 2. In the **Compose your message here...** field, type in the emoji code
 surrounded by colons. For example, `:smile:` produces the <img
 src="/static/generated/emoji/images/emoji/smile.png" alt="smile"

templates/zerver/help/at-mention-a-team-member.md

@@ -11,7 +11,7 @@ they may have already read the message.
     You can view any messages calling your attention by clicking on the
     **Mentions** filters in the left sidebar.
 
-1. Compose a [new message or reply](/help/reply-to-a-message).
+{!start-composing.md!}
 2. In the **Compose your message here...** field, type `@` followed by the
 email address or part of the username of the user you want to @-mention.
 When you begin typing an email address or username, autocomplete suggestions

templates/zerver/help/include/start-composing.md

@@ -0,0 +1 @@
+1. [Start composing](/help/compose-and-reply) a new message or reply.

templates/zerver/help/start-a-call.md

@@ -1,6 +1,6 @@
 # Start a video call
 
-1. Compose a [new message or reply](/help/reply-to-a-message).
+{!start-composing.md!}
 
 2. Click the video-camera (<i class="fa fa-video-camera"></i>) icon in the
 bottom left corner of the compose box. This will insert a link like