Add media requirements to docs

[mingard]

Notes on media fields:

 "leadImage": {
      "type": "Reference",
      "label": "Lead Image",
      "message": "JPEG or PNG",
      "required": false,
      "settings": {
        "collection": "mediaStore"
      },
      "publish": {
        "subType": "Image",
        "section": "Article",
        "placement": "sidebar",
        "display": {
          "list": false,
          "edit": true
        }
      }
    }

Notes on API settings requirements:

"media": {
    "enabled": true,
    "storage": "disk",
    "basePath": "workspace/media",
    "pathFormat": "date"
  }