TextBundle is still not a widely used format. And honestly, you can easily reproduce a similar thing by using a typical directory structure as you’d use with any web page. As an example…
Parent
-> Markdown Document
-> /Images
—> Img01.jpg
—> Img02.jpg
—> …
->/styles
—> styles.css
I use this structure every single day and it’s not only portable to iOS, but exportable. I can drag this to the Finder and have things resolve as expected.