help-docs: Move help center documentation to top level directory.

These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content, 
without it being unnecessary buried deep in the codebase.

help/mastering-the-compose-box.md

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+# Mastering the compose box
+
+When composing a message, Zulip lets you view a different conversation from the
+one you are composing to. For example, you can start a new topic without
+changing your narrow, send a private message about the topic you're viewing, or
+look up a related discussion.
+
+In this context, the parts of the message view that are outside of the
+conversation you are composing to are faded for clarity.
+
+## Go to conversation
+
+Zulip lets you narrow the message view to the conversation you're currently
+composing to.
+
+{start_tabs}
+
+{!start-composing.md!}
+
+1. If composing a stream message, click the **Go to conversation**
+   (<i class="zulip-icon zulip-icon-arrow-left-circle"></i>) button in the
+   compose box.
+
+!!! keyboard_tip ""
+
+    For both stream and private messages, use <kbd>Ctrl</kbd> + <kbd>.</kbd> to
+    go to the conversation you are composing to.
+
+{end_tabs}
+
+## Toggle between <kbd>Ctrl</kbd> + <kbd>Enter</kbd> and <kbd>Enter</kbd> to send a message
+
+By default, the <kbd>Enter</kbd> key adds a new line to your message,
+and <kbd>Ctrl</kbd> + <kbd>Enter</kbd> sends your message.
+
+This is convenient for typing multi-line messages, which are more common in
+Zulip than in most other chat products. However, you can also configure
+Zulip so that the <kbd>Enter</kbd> key sends your message.
+
+!!! tip ""
+
+    <kbd>Shift</kbd> + <kbd>Enter</kbd> always adds a new line, regardless
+    of whether **<kbd>Enter</kbd> to send** is enabled.
+
+### Enable <kbd>Enter</kbd> to send
+
+{start_tabs}
+
+{!start-composing.md!}
+
+1. Click **<kbd>Ctrl</kbd> + <kbd>Enter</kbd> to send** in the bottom right
+   corner of the compose box, just below the **Send** button.
+
+1. Select **<kbd>Enter</kbd> to send**.
+
+{end_tabs}
+
+### Enable **<kbd>Ctrl</kbd> + <kbd>Enter</kbd> to send**
+
+{start_tabs}
+
+{!start-composing.md!}
+
+1. Click **<kbd>Enter</kbd> to send** in the bottom right
+   corner of the compose box, just below the **Send** button.
+
+1. Select **<kbd>Ctrl</kbd> + <kbd>Enter</kbd> to send**.
+
+{end_tabs}
+
+## Related articles
+
+* [Resize the compose box](/help/resize-the-compose-box)
+* [Message formatting](/help/format-your-message-using-markdown)
+* [Preview messages before sending](/help/preview-your-message-before-sending)
+* [Messaging tips and tricks](/help/messaging-tips)