doc/contributing: update build instructions

We have moved away from Make since nixos-render-docs was introduced to
build the nixpkgs manual in NixOS#239636.

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

@@ -1,24 +1,12 @@
 # Contributing to this documentation {#chap-contributing}
 
-The sources of the Nixpkgs manual are in the [doc](https://github.com/NixOS/nixpkgs/tree/master/doc) subdirectory of the Nixpkgs repository. The manual is still partially written in DocBook but it is progressively being converted to [Markdown](#sec-contributing-markup).
+The sources of the Nixpkgs manual are in the [doc](https://github.com/NixOS/nixpkgs/tree/master/doc) subdirectory of the Nixpkgs repository.
 
-You can quickly check your edits with `make`:
+You can quickly check your edits with `nix-build`:
 
 ```ShellSession
-$ cd /path/to/nixpkgs/doc
-$ nix-shell
-[nix-shell]$ make
-```
-
-If you experience problems, run `make debug` to help understand the docbook errors.
-
-After making modifications to the manual, it's important to build it before committing. You can do that as follows:
-
-```ShellSession
-$ cd /path/to/nixpkgs/doc
-$ nix-shell
-[nix-shell]$ make clean
-[nix-shell]$ nix-build .
+$ cd /path/to/nixpkgs
+$ nix-build doc
 ```
 
 If the build succeeds, the manual will be in `./result/share/doc/nixpkgs/manual.html`.
@@ -114,5 +102,3 @@ Additional syntax extensions are available, all of which can be used in NixOS op
   >
   > watermelon
   > :   green fruit with red flesh
-
-For contributing to the legacy parts, please see [DocBook: The Definitive Guide](https://tdg.docbook.org/) or the [DocBook rocks! primer](https://web.archive.org/web/20200816233747/https://docbook.rocks/).