online image preview, screenshots update

[alex-cit]

fix #600 - Ability to preview online images

• online image preview using the attachment widget added to How to Embed an Image use case, HTML widget is now tagged as "advanced" (as I assume the attachment widget is more straightforward)
• public docs project https://app.merginmaps.com/projects/documentation/forms-display-images-and-files/tree updated to include this option. Now it refers to a pic from mergin maps blog - this picture should be maybe moved to the docs repository (so that URL doesn't change in the future), I put it to the write-docs section for now.
• screenshots updated for spacer widget and How to Open a File use case, to match the new project and slight changes in appearence of links in the mobile app

Note: the Integrated document viewer includes options to define width / height of the image, but this seems not to be working in the mobile app (should it work?).