Add a background to the transparent images for dark mode compatibility.

[katshann-raspberry]

No description provided.

[lurch]

Not directly related to this PR, but:

• all_audio_boards_gpio_pinouts.png probably doesn't need to be "W: 3556px | H: 2528px" ! (I guess that's probably a "print resolution" rather than a "web resolution"?)
• I'm not sure if it makes sense for the CM5-related technical drawings (cm4-cm5-antenna-physical.png, cm5-case-physical.png and cm5-cooler-physical.png) to be in our HTML docs; it might make more sense to just link to the relevant product-briefs instead? (Links can be found on https://www.raspberrypi.com/products/compute-module-4-antenna-kit/ https://www.raspberrypi.com/products/io-case-cm5/ and https://www.raspberrypi.com/products/cooler/ ). That way if the product-briefs get updated in future, there's less risk of the information in the HTML docs slipping out of date (IMHO).

[katshann-raspberry]

@lurch I've rescaled the audio image. The rest, I'll take as a separate piece of work.