doc/packages/treefmt: add function reference docs

Add a `treefmt.functionsDoc` passthru, which is included on the treefmt
section of the nixpkgs manual.

This generates reference docs for functions defined in
`pkgs/by-name/tr/treefmt/lib.nix`, using `nixdoc`.

doc/doc-support/package.nix

@@ -14,6 +14,7 @@
   nixpkgs ? { },
   markdown-code-runner,
   roboto,
+  treefmt,
 }:
 
 stdenvNoCC.mkDerivation (
@@ -47,6 +48,7 @@ stdenvNoCC.mkDerivation (
 
     postPatch = ''
       ln -s ${optionsJSON}/share/doc/nixos/options.json ./config-options.json
+      ln -s ${treefmt.functionsDoc.markdown} ./packages/treefmt-functions.section.md
     '';
 
     buildPhase = ''

doc/packages/treefmt.section.md

@@ -3,6 +3,10 @@
 [treefmt](https://github.com/numtide/treefmt) streamlines the process of applying formatters to your project, making it a breeze with just one command line.
 
 The [`treefmt` package](https://search.nixos.org/packages?channel=unstable&show=treefmt)
-provides functions for configuring treefmt using the module system.
+provides functions for configuring treefmt using the module system, which are [documented below](#sec-functions-library-treefmt).
 
 Alternatively, treefmt can be configured using [treefmt-nix](https://github.com/numtide/treefmt-nix).
+
+```{=include=} sections auto-id-prefix=auto-generated-treefmt-functions
+treefmt-functions.section.md
+```

doc/redirects.json

@@ -72,6 +72,15 @@
   "part-toolchains": [
     "index.html#part-toolchains"
   ],
+  "pkgs.treefmt.buildConfig": [
+    "index.html#pkgs.treefmt.buildConfig"
+  ],
+  "pkgs.treefmt.evalConfig": [
+    "index.html#pkgs.treefmt.evalConfig"
+  ],
+  "pkgs.treefmt.withConfig": [
+    "index.html#pkgs.treefmt.withConfig"
+  ],
   "preface": [
     "index.html#preface"
   ],
@@ -111,6 +120,9 @@
   "sec-building-packages-with-llvm-using-clang-stdenv": [
     "index.html#sec-building-packages-with-llvm-using-clang-stdenv"
   ],
+  "sec-functions-library-treefmt": [
+    "index.html#sec-functions-library-treefmt"
+  ],
   "sec-inkscape": [
     "index.html#sec-inkscape"
   ],