feat: Add "Using OpenAPI" module to course overview and structure

[SensibleWood]

User Story

As a student of OpenAPI
I want to understand how an OpenAPI document can be used from the perspective of an API consumer
So I can apply that knowledge in the context of my day-to-day job
And find appropriate tooling to help me

Detailed Requirement

The course was intended to provide a module combining API consumer and API provider views.

However, in writing the Creating OpenAPI Documents module its become readily apparent that the consumer-side of this need to be split out into a separate module.

Therefore, update the course overview with a new module "Using OpenAPI Documents" - which'll be wedged in between current 3 and 4 - and add the key points of the curriculum.