Converting portable text to markdown or HTML in Sanity page builder

// mypage.js

export default {
  name: 'mypage',
  type: 'document',
  title: 'mypage',
  fields: [
    {
      name: 'name',
      type: 'string',
      title: 'Name'
    },
    {
      name: 'slug',
      type: 'slug',
      title: 'Slug',
      description: 'Some frontends will require a slug to be set to be able to show the person',
      options: {
        source: 'name',
        maxLength: 96
      }
    },
    {
      name: 'pageBuilder',
      type: 'array',
      title: 'Page builder',
      of: [
        { type: 'hero' },
        { type: 'textWithIllustration' },
        { type: 'media' },
      ]
    }
  ]
}

 // media.js

export default {
	name: 'media',
	type: 'object',
	title: 'Media',
	fields: [
    {
			name: 'content',
			type: 'array',
			title: 'Content',
			of: [
				{
					type: 'block',
					title: 'Block',
					// Styles let you set what your user can mark up blocks with. These
					// corrensponds with HTML tags, but you can set any title or value
					// you want and decide how you want to deal with it where you want to
					// use your content.
					styles: [
						{ title: 'Normal', value: 'normal' },
						{ title: 'H1', value: 'h1' },
						{ title: 'H2', value: 'h2' },
						{ title: 'H3', value: 'h3' },
						{ title: 'H4', value: 'h4' },
						{ title: 'Quote', value: 'blockquote' }
					],
					lists: [{ title: 'Bullet', value: 'bullet' }, { title: 'Number', value: 'number' }],
					// Marks let you mark up inline text in the block editor.
					marks: {
						// Decorators usually describe a single property – e.g. a typographic
						// preference or highlighting by editors.
						decorators: [{ title: 'Strong', value: 'strong' }, { title: 'Emphasis', value: 'em' }],
						// Annotations can be any object structure – e.g. a link or a footnote.
						annotations: [
							{
								name: 'link',
								type: 'object',
								title: 'URL',
								fields: [
									{
										title: 'URL',
										name: 'href',
										type: 'url'
									}
								]
							}
						]
					},
					//of: [{ type: 'authorReference' }]
				},
				// You can add additional types here. Note that you can't use
				// primitive types such as 'string' and 'number' in the same array
				// as a block type.
				{
					type: 'mainImage',
					options: { hotspot: true }
				}
			]
		},
		{
			name: 'imageSize',
			type: 'string',
			title: 'Image Size',
			options: {
				list: [
					{ title: 'Small', value: 'small' },
					{ title: 'Large', value: 'large' }
				], // <-- predefined values
				layout: 'radio', // <-- defaults to 'dropdown'
			}
		},
		{
			name: 'imagePosition',
			type: 'string',
			title: 'Image Position',
			options: {
				list: [
					{ title: 'Start', value: 'start' },
					{ title: 'End', value: 'end' }
				], // <-- predefined values
				layout: 'radio', // <-- defaults to 'dropdown'
			}
		},
	],
}