feat: add array sidebar support

[kiaking]

This PR adds array type sidebar support. It only supports this syntax.

{
  sidebar: [
    {
      text: 'Getting Started',
      link: 'getting-started',
      children: [...]
    }
  ]
}

• You can't do sidebar: ['getting-started'].
• "Depth" option not supported. The items are always fully shown.
• Sidebar Groups works too, and header links works as well.
• Basic stylings applied, but not responsive design (not collapsable on mobile view).

[yyx990803]

Very niiice. I tweaked the font size a little bit.