nix: add page (#2808)

pages/common/nix.md

@@ -0,0 +1,23 @@
+# nix
+
+> Utilities for the Nix language and store.
+
+- Search for a package via its name or description:
+
+`nix search {{search_term}}`
+
+- Start a Nix shell with the specified packages available:
+
+`nix run {{nixpkgs.pkg1 nixpkgs.pkg2 nixpkgs.pkg3...}}`
+
+- Optimise Nix store disk usage by combining duplicate files:
+
+`nix optimise-store`
+
+- Start an interactive environment for evaluating Nix expressions:
+
+`nix repl`
+
+- Upgrade Nix to the latest stable version:
+
+`nix upgrade-nix`