Provide better documentation and examples for AppD auth support

[kriswest]

Area of Issue

[x] App Directory

Issue Description:

The App Directory supports optional authentication via a JWT token passed with API calls. However, documentation of authentication, including how an app should create or receive a valid token is at best light, leading to at least one firm assuming that authentication is not supported (#551_.

Provide more detailed documentation and examples, including improving the limited details in the AppD's usage page of what use-cases there are for authenticating access and how it might affect the behaviour of/responses from the AppD.