refactor(docs): switch to nixos-render-docs for nix options generation

- Use nixos-render-docs commonmark as the rendering engine
- Add GitHub source links (Declared by) for each option
- Fix installation.md links pointing to /docs/nix-module -> /docs/nix-options

nix/generate-options.nix

@@ -6,6 +6,9 @@ self:
   optionPrefix,
 }:
 let
+  # Absolute store path of the flake root, used to compute relative subpaths
+  repoPath = toString self;
+
   eval = lib.evalModules {
     modules = [
       (import module self)
@@ -14,6 +17,15 @@ let
     specialArgs = { inherit pkgs; };
   };
 
+  # Relative path of the module file within the repo (e.g. "nix/hm-modules.nix")
+  moduleSubpath = lib.removePrefix "/" (lib.removePrefix repoPath (toString module));
+
+  # Declaration entry linking each option back to its source file on GitHub
+  moduleDeclaration = {
+    url = "https://github.com/mangowm/mango/blob/main/${moduleSubpath}";
+    name = "<mango/${moduleSubpath}>";
+  };
+
   optionsDoc = pkgs.nixosOptionsDoc {
     options = eval.options;
     transformOptions =
@@ -21,7 +33,9 @@ let
       opt
       // {
         visible = opt.visible && !opt.internal;
+        # Strip the option prefix so docs show "enable" instead of "programs.mango.enable"
         name = lib.removePrefix optionPrefix opt.name;
+        declarations = [ moduleDeclaration ];
       };
   };
 in