feat(v2): allow non sidebar category to be first item of sidebar

[endiliey]

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"

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[docusaurus-bot]

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Built with commit 0b931e4

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[yangshun]

Nice tests

[yangshun]

Do we not need to check anymore?

[endiliey]

if the first item is not a category, items is undefined. Checking is alrd done in the DocSidebarItem itself

[endiliey]

without this loc removal, it will error

[yangshun]

Ok please resolve conflicts and self-merge :D

[endiliey]

Yeah that will have to wait maybe tonight. I have plans for today

[flaurida]

thank you! 🎉