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Furthermore, the links to minimal and target images descriptions on the website have been superseded by the fixedfunction and hmi images, so we make sure we point to a valid link.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.