[Docs] Update iframe examples to add feature policy for camera access

[Giwayume]

This is one of many files that show how to instantiate minpaint on another page using an iframe:
https://github.com/viliusle/miniPaint/blob/master/examples/add-edit-imgData.html#L4
I don't know the list of all files that show iframe usage examples.

When the user does File -> Open -> Open from Webcam, this feature has usage limiations. By default access to the webcam from iframes is not allowed.

For iframes that need this access, need to add allow="camera" to the iframe declaration.

<iframe allow="camera"></iframe>

[viliusle]

I will update examples.