Experimental MDX field does not support captions for images

[sapphies]

I noticed that on the experimental MDX field, Keystatic doesn't support captions for images.

So you would have something like this being displayed in Keystatic's editor: ![Alt text](/path/to/img.jpg "Caption text")

I added the image by manually editing the file. However if you add an image with a caption via Keystatic's WYSIWYG editor... something weird happens if you then inspect the Markdown file.