reduce to minimal proposal

based on https://github.com/NixOS/nix.dev/issues/275#issuecomment-1227572104

@edolstra proposed using "Nixpkgs" as a proper name

- this is not pronouncible from just reading it, but at least it's brief
  and unambiguous
  -> added pronounciation

@uningan pointed out Nix is more than just a package manager, but also
a built system.

- this is true, and probably we don't have to reflect that in the proper
  name
- also don't have to change anything if we keep Nix
  -> back to just Nix

multiple threads of discussion diverged into very broad proposals to
rename things, but the focus here should be on the smallest possible change to
improve clarity and get consensus on something at all.
  -> removed additional proposals, which are tangential

CONTRIBUTING.md

@@ -188,36 +188,32 @@ Collect links at the end of each section, which are delimited by headings.
 
 To avoid confusion around all the things called Nix, always use the following terms with capitalization as given.
 
-Terms in brackets (`[  ]`) are optional.
-Terms in parentheses (`(  )`) are abbreviations.
+- Nix
 
-- Nix ecosystem
+  Build system and package manager.
 
-  All of the below.
+  Read /nɪks/ ("Niks").
 
-  - Nix package manager
+- Nix language
 
-    - `nix` [command [line interface]] (Nix CLI)
+    Programming language to declare packages and configurations for Nix.
 
-  - Nix language
+  - Nix expression
 
-      Programming language to declare packages and configurations for the Nix package manager.
+    Expression written in Nix language.
 
-    - Nix expression
+  - Nix file
 
-      Expression written in Nix language.
+    File (`.nix`) containing a Nix expression.
 
-    - Nix file
+- Nixpkgs
 
-      File containing Nix expression, usally ends in `.nix`.
+  Software distribution built with Nix.
 
-  - Nix package collection (`nixpkgs`)
+  Read /nɪks ˈpækɪʤɪz/ ("Niks packages").
 
-    Read `nixpkgs` as "Nix packages".
+- NixOS
 
-  - NixOS [Linux distribution]
+  Linux distribution based on Nix and Nixpkgs.
 
-    Linux distribution based on Nix package manager and `nixpkgs`.
-    Read /nɪks oʊ ɛs/ ("Nix Oh Es").
-
-    - NixOS module collection (NixOS modules)
+  Read /nɪks oʊ ɛs/ ("Niks Oh Es").