refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/development/nixos-tests.chapter.md

@@ -0,0 +1,14 @@
+# NixOS Tests {#sec-nixos-tests}
+
+When you add some feature to NixOS, you should write a test for it.
+NixOS tests are kept in the directory `nixos/tests`, and are executed
+(using Nix) by a testing framework that automatically starts one or more
+virtual machines containing the NixOS system(s) required for the test.
+
+```{=include=} sections
+writing-nixos-tests.section.md
+running-nixos-tests.section.md
+running-nixos-tests-interactively.section.md
+linking-nixos-tests-to-packages.section.md
+testing-hardware-features.section.md
+```