doc(p(
        text("Hello "),
        text("world").strong()
));

{
  "version": 1,
  "type": "doc",
  "content": [
    {
      "type": "paragraph",
      "content": [
        {
          "type": "text",
          "text": "Hello "
        },
        {
          "type": "text",
          "text": "world",
          "marks": [
            {
              "type": "strong"
            }
          ]
        }
      ]
    }
  ]
}