Document custom content element icons

[nhovratov]

Hey, thank you. Actually there is a documentation for that, but probably a little bit hidden: https://docs.typo3.org/p/mask/mask/8.3/en-us/BackendPreview/Index.html#preview-image-for-a-content-element

Let's just add a link here to this part of the documentation, so we don't have it twice.

[cweiske]

I've updated the patch.