api-docs: Add article about markup message formatting.

Adds a "Message formatting" article to the API documentation. To
start, the article is a changelog of updates to markup features for
messages, with the idea being that it can be expanded to further
describe and document Zulip's markup features in future iterations.

api_docs/changelog.md

@@ -1795,10 +1795,10 @@ field with an integer field `invite_to_realm_policy`.
 
 **Feature level 33**
 
-* Markdown code blocks now have a `data-code-language` attribute
-  attached to the outer `div` element, recording the programming
-  language that was selecting for syntax highlighting.  This field
-  supports the upcoming "view in playground" feature for code blocks.
+* [Markdown message formatting](/api/message-formatting#code-blocks):
+  [Code blocks](/help/code-blocks) now have a `data-code-language`
+  attribute attached to the outer HTML `div` element, recording the
+  programming language that was selected for syntax highlighting.
 
 **Feature level 32**
 
@@ -1854,9 +1854,9 @@ No changes; feature level used for Zulip 3.0 release.
 
 **Feature level 24**
 
-* The `!avatar()` and `!gravatar()` Markdown syntax, which was never
-  documented, had inconsistent syntax, and was rarely used, was
-  removed.
+* [Markdown message formatting](/api/message-formatting#removed-features):
+  The rarely used `!avatar()` and `!gravatar()` markup syntax, which
+  was never documented and had inconsistent syntax, was removed.
 
 **Feature level 23**
 
@@ -1921,8 +1921,8 @@ No changes; feature level used for Zulip 3.0 release.
 
 **Feature level 15**
 
-* Added [spoilers](/help/format-your-message-using-markdown#spoilers)
-  to supported Markdown features.
+* [Markdown message formatting](/api/message-formatting#spoilers): Added
+  [spoilers](/help/spoilers) to supported message formatting features.
 
 **Feature level 14**
 
@@ -1984,8 +1984,9 @@ No changes; feature level used for Zulip 3.0 release.
 * [`GET /users`](/api/get-users), [`GET /users/{user_id}`](/api/get-user)
   and [`GET /users/me`](/api/get-own-user): User objects now contain the
   `is_owner` field as well.
-* Added [time mentions](/help/format-your-message-using-markdown#mention-a-time)
-  to supported Markdown features.
+* [Markdown message formatting](/api/message-formatting#global-times):
+  Added [global times](/help/global-times) to supported message
+  formatting features.
 
 **Feature level 7**
 

api_docs/message-formatting.md

@@ -0,0 +1,47 @@
+# Message formatting
+
+Zulip supports an extended version of Markdown for messages, as well as
+some HTML level special behavior. The Zulip help center article on [message
+formatting](/help/format-your-message-using-markdown) is the primary
+documentation for Zulip's markup features. This article is currently a
+changelog for updates to these features.
+
+The [render a message](/api/render-message) endpoint can be used to get
+the current HTML version of any Markdown syntax for message content.
+
+## Code blocks
+
+**Changes**: As of Zulip 4.0 (feature level 33), [code blocks][help-code]
+can have a `data-code-language` attribute attached to the outer HTML
+`div` element, which records the programming language that was selected
+for syntax highlighting. This field is used in the
+[playgrounds][help-playgrounds] feature for code blocks.
+
+## Global times
+
+**Changes**: In Zulip 3.0 (feature level 8), added [global time
+mentions][help-global-time] to supported Markdown message formatting
+features.
+
+## Spoilers
+
+**Changes**: In Zulip 3.0 (feature level 15), added
+[spoilers][help-spoilers] to supported Markdown message formatting
+features.
+
+## Removed features
+
+**Changes**: In Zulip 4.0 (feature level 24), the rarely used `!avatar()`
+and `!gravatar()` markup syntax, which was never documented and had an
+inconsistent syntax, were removed.
+
+## Related articles
+
+* [Markdown formatting](/help/format-your-message-using-markdown)
+* [Send a message](/api/send-message)
+* [Render a message](/api/render-message)
+
+[help-code]: /help/code-blocks
+[help-playgrounds]: /help/code-blocks#code-playgrounds
+[help-spoilers]: /help/spoilers
+[help-global-time]: /help/global-times

api_docs/sidebar_index.md

@@ -21,6 +21,7 @@
 * [HTTP headers](/api/http-headers)
 * [Error handling](/api/rest-error-handling)
 * [Roles and permissions](/api/roles-and-permissions)
+* [Message formatting](/api/message-formatting)
 * [Client libraries](/api/client-libraries)
 * [API changelog](/api/changelog)