Sidebar example and documentation

[StefMa]

The sidebar option is not documented nor showed as an example.

Add the option to the readme and to the example site.

[StefMa]

This is how an sidebar can be configured:

  sidebar:
    logo: fresh-square
    sections:
    - title: User
      icon: user
      links:
      - text: Profile
        url: /
      - text: Account
        url: /
      - text: Settings
        url: /
    - title: More
      icon: another-icon
      links:
      - text: Special More
        url: /
      - text: Even More
        url: /