refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/configuration/config-syntax.chapter.md

@@ -0,0 +1,18 @@
+# Configuration Syntax {#sec-configuration-syntax}
+
+The NixOS configuration file `/etc/nixos/configuration.nix` is actually
+a *Nix expression*, which is the Nix package manager's purely functional
+language for describing how to build packages and configurations. This
+means you have all the expressive power of that language at your
+disposal, including the ability to abstract over common patterns, which
+is very useful when managing complex systems. The syntax and semantics
+of the Nix language are fully described in the [Nix
+manual](https://nixos.org/nix/manual/#chap-writing-nix-expressions), but
+here we give a short overview of the most important constructs useful in
+NixOS configuration files.
+
+```{=include=} sections
+config-file.section.md
+abstractions.section.md
+modularity.section.md
+```