You can include both document content and any external HTML code into the note using [include path/url] tag:
[include document_path] will include any given document with all its child includes. Please note that you can include a document just once in order to avoid circular references. The documents you include will come in a read-only mode to avoid confusion when you edit their masters.
[include url] downloads any HTML content from a given url and includes it into the note when anybody views it. This way you can embed your code written in any language into the Markout notebooks. You can even include external forms but don't forget to provide absolute urls for their actions.