Docs - Missing Step from Delivery API documentation

There is a missing step in the Delivery API documentation to enable the delivery API.  It's caught me out a couple of times when setting up a new site.  I think its worthwhile to include it.

https://github.com/prjseal/Clean/blob/main/.github/clean-headless-api.md

builder.CreateUmbracoBuilder()
    .AddBackOffice()
    .AddWebsite()
    **.AddDeliveryApi()**
    .AddComposers()
    .Build();


