Full dev environment based on pixi-only

This MR introduces a CI pipeline that is based exclusively on the use of pixi. Furthermore:

• The CI file is completely self-contained and has no external dependencies on software/idiap-citools>
• Development does not require the use of software/idiap-devtools> anymore and relies exclusively on pixi - lock files assume the role of dependency pinning that was previously carried out by our own biosignal/software/dev-profile>.
• We now use more recent versions pytorch/torchvision from the pytorch channel
• A recipe to run on a CUDA-enabled machine alongside instructions are provided
• Documentation is updated to reflect these changes.