Update and clarify the image downloads page for v2022

The previous layout assumed that v2021, and its image types, were stable, and the new image types (FixedFunction and HMI) were development-only. As a result, several of the links were broken, and the relation between image releases and the overall page layout was rather confusing.

https://phabricator.apertis.org/T8593

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com