doc: collapse platforms and modules

Collapsing is only possible for pages, not headings, so I had to
combine platforms and modules into a single section and nest them
under draft pages.

Later, we could migrate some content from the configuration page to
these draft pages to make the flow of the book more natural.

docs/book.toml

@@ -9,3 +9,6 @@ site-url = "/stylix/"
 git-repository-url = "https://github.com/danth/stylix"
 edit-url-template = "https://github.com/danth/stylix/edit/master/docs/{path}"
 no-section-label = true
+
+[output.html.fold]
+enable = true

docs/default.nix

@@ -45,7 +45,7 @@ let
     }).optionsCommonMark;
 
   # The documentation for options which aren't linked to a specific module
-  makeGlobalOptionsDoc =
+  makePlatformsOptionsDoc =
     configuration:
     makeOptionsDoc {
       inherit configuration;
@@ -159,9 +159,9 @@ pkgs.stdenvNoCC.mkDerivation {
     cp ${../gnome.png} src/gnome.png
     cp ${../kde.png} src/kde.png
 
-    mkdir --parents src/options/global
-    writeOptions 'Home Manager' ${(makeGlobalOptionsDoc homeManagerConfiguration)} src/options/global/home_manager.md
-    writeOptions 'NixOS' ${(makeGlobalOptionsDoc nixosConfiguration)} src/options/global/nixos.md
+    mkdir --parents src/options/platforms
+    writeOptions 'Home Manager' ${(makePlatformsOptionsDoc homeManagerConfiguration)} src/options/platforms/home_manager.md
+    writeOptions 'NixOS' ${(makePlatformsOptionsDoc nixosConfiguration)} src/options/platforms/nixos.md
 
     mkdir --parents src/options/modules
     ${modulePageScript}

docs/src/SUMMARY.md

@@ -14,11 +14,15 @@
 - [Testbeds](testbeds.md)
 - [Style guide](styling.md)
 
-# Platforms
+# Reference
 
-- [Home Manager](options/global/home_manager.md)
-- [NixOS](options/global/nixos.md)
+<!--
+  The auto-generated list of modules is appended to this file, so this must
+  be the last section, and modules must be the last page, with no comments
+  following it. There must be a trailing newline.
+-->
 
-# Modules
-
-<!-- The auto-generated list of modules will be added below. -->
+- [Platforms]() <!-- TODO: migrate general platforms content to this page. -->
+  - [Home Manager](options/platforms/home_manager.md)
+  - [NixOS](options/platforms/nixos.md)
+- [Modules]() <!-- TODO: migrate general modules content to this page. -->