feat: add function to generate docs for nixlet values

add docs for nixlets in this repo to the mkdocs site

lib/valuesDocs.nix

@@ -0,0 +1,84 @@
+{
+  lib,
+  nixlet,
+  transformOptions ? opt: opt,
+  filter ? _: true,
+  headingDepth ? 3,
+  ...
+}: let
+  inherit
+    (lib)
+    removeSuffix
+    concatStringsSep
+    mapAttrsToList
+    concatStrings
+    replicate
+    ;
+
+  _transformOptions = opt:
+    transformOptions (opt
+      // {
+        visible = let
+          filtered = !builtins.elem (builtins.head opt.loc) ["_module"];
+        in
+          filtered && opt.visible && (filter opt);
+        name = lib.removePrefix "config." opt.name;
+      });
+
+  rawOpts = lib.optionAttrSetToDocList nixlet.values.options;
+  transformedOpts = map _transformOptions rawOpts;
+  filteredOpts = lib.filter (opt: opt.visible && !opt.internal) transformedOpts;
+
+  optionsNix = builtins.listToAttrs (
+    map (o: {
+      name = o.name;
+      value = removeAttrs o [
+        "visible"
+        "internal"
+      ];
+    })
+    filteredOpts
+  );
+
+  optToMd = opt: let
+    headingDecl = concatStrings (replicate headingDepth "#");
+  in
+    ''
+      ${headingDecl} `${opt.name}`
+
+      ${
+        if opt.description != null
+        then opt.description
+        else "(no description)"
+      }
+
+      **Type**:
+
+      ```console
+      ${opt.type}
+      ```
+    ''
+    + (lib.optionalString (opt ? default && opt.default != null) ''
+
+      **Default value**:
+
+      ```nix
+      ${removeSuffix "\n" opt.default.text}
+      ```
+    '')
+    + (lib.optionalString (opt ? example) ''
+
+      **Example value**:
+
+      ```nix
+      ${removeSuffix "\n" opt.example.text}
+      ```
+    '')
+    + "\n";
+
+  opts = mapAttrsToList (name: opt:
+    optToMd opt)
+  optionsNix;
+  markdown = concatStringsSep "\n" opts;
+in
+  builtins.toFile "values-doc.md" markdown