feat(v2): allow user to customize/enhance the default sidebar items generator

[slorber]

Motivation

Customize the sidebar items generator logic by reusing and enhancing the default sidebar items generator with yur own logic.

Fixes #4640

Here's an example to generate a sidebar in reverse order:

// Reverse the sidebar items ordering (including nested category items)
function reverseSidebarItems(items) {
  // Reverse items in categories
  const result = items.map(item => {
    if (item.type === "category") {
      return { ...item, items: reverseSidebarItems(item.items) };
    }
    return item;
  });
  // Reverse items at current level
  result.reverse();
  return result;
}

module.exports = {
  plugins: [
    [
      "@docusaurus/plugin-content-docs",
      {
        sidebarItemsGenerator: async function({
          defaultSidebarItemsGenerator,
          ...args
        }) {
          const sidebarItems = await defaultSidebarItemsGenerator(args);
          return reverseSidebarItems(sidebarItems);
        }
      }
    ]
  ]
};

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests

[netlify]

[V1]

Built with commit 67ab5b0

https://deploy-preview-4658--docusaurus-1.netlify.app

[netlify]

[V2]

Built with commit 67ab5b0

https://deploy-preview-4658--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	80
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4658--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 630 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.5 kB	0 B
website/build/assets/js/main.********.js	453 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.3 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	27.1 kB	0 B

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