add markdown codeblock renderer extension point (#1492)

Introduces a new extension point that allows developers to overwrite the default syntax highlighting renderer for specific code blocks defined in markdown. The extension point is dynamic and follows the pattern "markdown-renderer.code.{language}".
This feature lays the groundwork for the scm-markdown-plantuml-plugin.

docs/en/development/plugins/extension-points.md

@@ -57,7 +57,10 @@ The following extension points are provided for the frontend:
 ### roles.route
 ### user.route
 ### user.setting
-
+### markdown-renderer.code.{language}
+- Dynamic extension point for custom language-specific renderers
+- Overrides the default Syntax Highlighter
+- Used by the Markdown Plantuml Plugin
 
 # Deprecated