dGC "Subscriber Guide" section in dGC docs needs developing

[pacharanero]

Some notes for when doing next review of the docs site:

• Following conversations with some of our customers it may help to have an architecture diagram showing how the dGC API fits in with their current system - needs to be clearer that the base EPR is the one calling our API.
• Possibly need a clearer walkthrough of how to creat an App in the Dev Portal
• Need to add a section showing how to view reporting/usage in the Dev Portal
• FAQs section - full review and add in questions asked by the NHS cBMI team.