Document how to create a new extension type

[t6d]

We need to document how to introduce a new extension type (possibly even with a new surface area). In short, these are the necessary steps:

1. Add the new extension type to the list of extensions that can use the new development server in Shopify CLI,
2. Whitelist the renderer that is used by the extension in Shopify CLI,
3. Add a new template in Shopify CLI Extensions, and optionally
4. Introduce a new surface area in Shopify CLI.

Ideally this information should be available here: https://extensions.shopifycloud.com/specifications/new

[dmhenry]

PR: https://github.com/Shopify/extensions/pull/1425

See https://extensions.shopifycloud.com/specifications/new or corresponding Google Doc