create-rpi-oslist: use more explicit descriptions
The image type (minimal, fixedfuntion...) might not be clear enough
for people wanting to give Apertis a try to clearly understand what to
expect from each image. In order to make things clearer, this commit
adjust image descriptions by using the terms headless and graphical
which can be more easily understood (that way, we also stay consistent
with how the daily oslist is generated).
Furthermore, the links to minimal and target images descriptions on
the website have been superseded by the fixedfunction and hmi
images, so we make sure we point to a valid link.
This MR also fixes the image URLs.