[Change Request] make images look good in dark mode for all chapters

[CunliangGeng]

Some images in the chapters use transparent background, which does not make the image look in dark mode as good as in light mode, for example, the v6 infrastructure image in chapter 2.

Every chapter author should check their chapters and update the images to make them look good on both light and dark modes.

The following chapters have been checked/updated:

• setup
• ch 1
• ch 2
• ch 3
• ch 4
• ch 5
• ch 6
• ch 7
• ch 8