docs/dev/flake.md (new): the layout of the one flake, the naming rule, the table of every old
attribute (root flake, kfde-, kfm- and the nested flakes) and its new name, the inputs and
why there are two nixpkgs (x86_64-darwin only), how the source of each platform is cut and why
the stubs are needed, the check groups (the NixOS/VM checks are heavyChecks until the structure
of the check sets is decided), the dev shells, what to run after an update.
docs/dev/build-and-test.md: the nix commands use windows-*, linux-*, macos-* names, the final
directories and legacyPackages.<s>.vm.NAME for a VM suite.
Verified: not run as prose; the attribute names were evaluated by nix flake check --no-build
--all-systems and the Windows ones built (see the flake revision).
