feat(theme-classic, plugin-docs): sidebar item level-specific className + allow customization (#5642)

* Initial work

* Complete function

* Avoid duplication

* More dedupe

* Make everything constants

* Change casing & docs

website/_dogfooding/docs-tests-sidebars.js

@@ -10,6 +10,7 @@ module.exports = {
     {
       type: 'doc',
       id: 'index',
+      className: 'red',
       label: 'Index',
     },
     {
@@ -23,9 +24,16 @@ module.exports = {
       label: 'Huge sidebar category',
       items: generateHugeSidebarItems(4),
     },
+    {
+      type: 'link',
+      label: 'External link',
+      href: 'https://github.com/facebook/docusaurus',
+      className: 'red',
+    },
     {
       type: 'category',
       label: 'TOC tests',
+      className: 'red',
       items: [
         {
           type: 'autogenerated',