Optimise image and YouTube frame loading

[MichaIng]

We should lazy load images on the docs so that they are loaded only when being scrolled to. Not all but modern browsers support it: https://squidfunk.github.io/mkdocs-material/reference/images/#image-lazy-loading
What we could additional take care of is adding the full image dimensions (not only width, but height as well), so that content shift is avoided.

The YouTube videos added a lot of JavaScript to the pages. Let's check if it is possible to lazy-load those as well, or if there are other ways, e.g. enabling the whole frame only on click or such. Also I'd like to review the client features we currently allow it to use, like accelerometer, autoplay, gyroscope, where I see no reason to allow access to such sensors, and definitely not autoplay 😉. My dream would be that the whole frame is completely unloaded until a simple link button is pressed, so no YouTube asset is loaded until interactive action.

[MichaIng]

Finished, merged with next release: #401