nix: Move docs tools into core infrastructure

docs/README.md

@@ -2,17 +2,7 @@
 
 PostgREST docs use the reStructuredText format, check this [cheatsheet](https://github.com/ralsina/rst-cheatsheet/blob/master/rst-cheatsheet.rst) to get acquainted with it.
 
-To build the docs locally, use [nix](https://nixos.org/nix/):
-
-```bash
-  nix-shell
-```
-
-Once in the nix-shell you have the following commands available:
-
-- `postgrest-docs-build`: Build the docs.
-- `postgrest-docs-serve`: Build the docs and start a livereload server on `http://localhost:5500`.
-- `postgrest-docs-spellcheck`: Run aspell.
+To build the docs locally, see [the Nix development readme](/nix/README.md#documentation).
 
 ## Documentation structure