Motivation

This allows user to define sidebar item as the first level item of a sidebar. If previously its defined like this

type Sidebar = {
  [sidebarId: string]: {
    [sidebarCategory: string]: SidebarItem[];
  }
}

Its now

type Sidebar = {
  [sidebarId: string]: {
    [sidebarCategory: string]: SidebarItem[];
  } | SidebarItem[];
}

Other changes

• I added a lot of other tests in sidebars. So now if there is any wrong type of 'label', 'items', unknown type and so on, user will receive MUCH BETTER error messages. This is still 100% test coverage on docs
  Updated babel so i can use TS 3.7 assertion
• Docs updated

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

// sidebars.js
module.exports = {
    docs: [
    {
      type: 'category',
      label: 'Docusaurus',
      items: ['introduction', 'design-principles', 'contributing'],
    },
    {
      type: 'category',
      label: 'Getting Started',
      items: ['installation', 'configuration'],
    },
    'creating-pages',
    'cli',
     {
        type: 'category',
        label: 'Plugins',
        items: ['using-plugins', 'advanced-plugins'],
      },
  ],
};

Here's a cool demo showing it. You can directly click "Creating Pages"