doc: collapse old contributing sections to one redirect

also link from nixos/doc to the in-tree README directly

doc/contributing/contributing-to-documentation.chapter.md

@@ -1,11 +1,3 @@
 # Contributing to Nixpkgs documentation {#chap-contributing}
 
 This section has been moved to [doc/README.md](https://github.com/NixOS/nixpkgs/blob/master/doc/README.md).
-
-## devmode {#sec-contributing-devmode}
-
-This section has been moved to [doc/README.md](https://github.com/NixOS/nixpkgs/blob/master/doc/README.md).
-
-## Syntax {#sec-contributing-markup}
-
-This section has been moved to [doc/README.md](https://github.com/NixOS/nixpkgs/blob/master/doc/README.md).

doc/redirects.json

@@ -4663,12 +4663,8 @@
     "index.html#reviewing-contributions--merging-pull-requests"
   ],
   "chap-contributing": [
-    "index.html#chap-contributing"
-  ],
-  "sec-contributing-devmode": [
-    "index.html#sec-contributing-devmode"
-  ],
-  "sec-contributing-markup": [
+    "index.html#chap-contributing",
+    "index.html#sec-contributing-devmode",
     "index.html#sec-contributing-markup"
   ],
   "part-interoperability": [

nixos/doc/manual/contributing-to-this-manual.chapter.md

@@ -1,7 +1,7 @@
 # Contributing to this manual {#chap-contributing}
 
 The sources of the NixOS manual are in the [nixos/doc/manual](https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual) subdirectory of the [Nixpkgs](https://github.com/NixOS/nixpkgs) repository.
-This manual uses the [Nixpkgs manual syntax](https://nixos.org/manual/nixpkgs/unstable/#sec-contributing-markup).
+This manual uses the [Nixpkgs manual syntax](https://github.com/NixOS/nixpkgs/blob/master/doc/README.md#syntax).
 
 You can quickly check your edits with the following: