Update navigation title in Markdown extensions doc

The `title_navigation` field was changed from "Markdown Extensions" to "Markdown Syntax" for clarity and consistency. This aligns the navigation label with the document's content and improves user experience.

Author: Mpdreamz

docs/source/syntax/md-extensions.md

@@ -1,6 +1,6 @@
 ---
 title: Markdown Syntax Extensions
-title_navigation: Markdown Extensions
+title_navigation: Markdown Syntax
 ---
 
 ## Syntax