fix: update project from devenv to nix flake

documentation/manual/de/book/theme-tomorrow_night.js

documentation/manual/de/book/tomorrow-night.css

documentation/manual/de/mdbook-admonish.css

documentation/manual/de/mermaid-init.js

documentation/manual/de/mermaid.min.js

documentation/manual/de/source/01-Einleitung.md

documentation/manual/de/source/02-Erste-Schritte.md

documentation/manual/de/source/404.md

documentation/manual/de/source/SUMMARY.md

documentation/manual/de/source/imprint.md

documentation/manual/de/theme/book.js

flake.lock

flake.nix

go.mod

go.sum

nix/config/common-packages.nix

nix/config/defaults.nix

nix/config/release.nix

nix/config/release.nix.template

nix/packages/manual.nix