Update boot folder documentation to match latest changes for bookworm…

[nathan-contino]

• closes Document Boot Folder changes #3153
• miscellaneous cleanup while I'm in this part of the docs, including eliminating an entire header in this section that didn't contribute any useful hierarchy

[vinceskahan]

I understand what you did and did read the linked page re: headless setup via the imaging process at https://www.raspberrypi.com/documentation/computers/configuration.html#set-up-a-headless-raspberry-pi but still feel this is extremely inadequate versus how things were set up in previous deb-11 and earlier os.

But I'm going to stop asking because it is never going to happen.

Your project clearly is not going to make it easy for folks to set up a wireless-only network-enabled headless pi without using your rpi imager tool. Very unhappy with that decision and it's a big step backward, but your project seems set in its ways.

Thank you for at least documenting the as-is FWIW.