rock64, rockpro64, raspberrypi4b: Add a link to the hardware page on the wiki and add the necessary details there

For each of the boards using the generic arm64 image, we have a page ("point A") with download links:

• https://freedombox.org/download/rock64/
• https://freedombox.org/download/rockpro64/
• https://freedombox.org/download/raspberrypi4b/

Currently, they only link to https://wiki.debian.org/FreedomBox/Download.

But the instructions on how to actually use these images are on the device-specific pages ("point B"):

• https://wiki.debian.org/FreedomBox/Hardware/Rock64
• https://wiki.debian.org/FreedomBox/Hardware/RockPro64
• https://wiki.debian.org/FreedomBox/Hardware/RaspberryPi4B

Currently, there is no obvious path to get from "point A" to "point B". The task is to create this path.

The approach I suggested, when originally creating this issue, is to simply add a link to the wiki's Hardware pages for the appropriate board. However, this may not be the best approach, feel free to propose an alternative.