Update RPi reference board information

The documentation for the Raspberry Pi 4 states that support through the
RPi Imager is ongoing, however this seems to have been completed a few
years ago. The links to download images are also broken.

Update the page to have suitable links and to mention that the RPi
Imager can be used.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>