text: Title: base64 and tracedSVG are not part of ImageObjectType
  Body: Using the guide, it does not seem that Base64 or SVG exists within the object that defines images types. The documentation section in question is linked here: https://wagtail-grapple.readthedocs.io/en/latest/general-usage/graphql-types.html?highlight=tracedSvg%3A%20String%20base64%3A#imageobjecttype
  
  The query:
  ``` GraphQl
  {
    pages {
      ... on ExamplePage {
        title
        image {
          src
          base64
        } 
      }
    }
  }
  ```
  
  The output:
  ``` GraphQl
  {
    "errors": [
      {
        "message": "Cannot query field \"base64\" on type \"ImageObjectType\".",
        "locations": [
          {
            "line": 7,
            "column": 9
          }
        ]
      }
    ]
  }
  ```
  
  I believe that this is inhibiting use with [Wagtail-Gatsby](https://github.com/NathHorrigan/wagtail-gatsby)