[Feature] Allow using multiple sidebars in the sidebar config file when using --sidebar/-sb flag

[lmpham1]

Currently, the program only supports one sidebar object per config file, i.e.:

{
    "sidebar": {
        "type": "pages",
        "title": "sample stories",
        "items": "./sample"
    }
}

The program should be to support multiple sidebars in the config file, similar to how Docusaurus does it:

module.exports = {
  mySidebar: [
    {
      type: 'category',
      label: 'Getting Started',
      items: ['doc1'],
    },
    {
      type: 'category',
      label: 'Docusaurus',
      items: ['doc2', 'doc3'],
    },
  ],
};

Note that they have an array of two sidebars in the snippet above