[Feature Request] Add iframe previews for tutorials list

## User request

Currently the official community tutorials list in the documentation section only contains links to videos and not thumbnail previews of the video links themselves.

I am aware that Github Pages does not support iframes, hence no displaying of thumbnails, but with a little help from a tool service such as: https://www.get-youtube-thumbnail.com/
Manual video previews could be implemented with a bit of work.

## Reference
Docs: https://doc.babylonjs.com/guidedLearning/offsite
Github: https://github.com/BabylonJS/Documentation/blob/master/content/toolsAndResources/offsite.md