Draft: feat: V3

• switch to Nix module system completely
• more modular and also more granular "typing" of the CI yaml keywords
• switch to more lightweight rensa nix framework + friends, but still support flake-parts in the lib
• add Soonix integration which can automatically generate the .gitlab-ci.yml for you, with correct version matching the current flake version
• improve docs, update nixmkdocs etc.

Closes #26 (closed)