docs: nix formatter (#6414)

Co-authored-by: opencode-agent[bot] <opencode-agent[bot]@users.noreply.github.com>

packages/web/src/content/docs/formatters.mdx

@@ -30,6 +30,7 @@ OpenCode comes with several built-in formatters for popular languages and framew
 | ocamlformat          | .ml, .mli                                                                                                | `ocamlformat` command available and `.ocamlformat` config file                                        |
 | terraform            | .tf, .tfvars                                                                                             | `terraform` command available                                                                         |
 | gleam                | .gleam                                                                                                   | `gleam` command available                                                                             |
+| nixfmt               | .nix                                                                                                     | `nixfmt` command available                                                                            |
 | shfmt                | .sh, .bash                                                                                               | `shfmt` command available                                                                             |
 | oxfmt (Experimental) | .js, .jsx, .ts, .tsx                                                                                     | `oxfmt` dependency in `package.json` and an [experimental env variable flag](/docs/cli/#experimental) |