Add documentation around api payloads

[ianco]

The payloads for some api endpoints (for example /v4/topic/{id}/credential-set) are fairly complicated and difficult to interpret for someone who is not familiar with the orgbook data model and how credentials are issued, received and processed.

Add some documentation to this site to explain:

• the core orgbook credentials (registration, business number, relationship)
• how credential history works
• how data is presented in payloads and how to look up the credential attribute values
• how to request active creds only, history, etc
• how to use id's from one endpoint as a filter for another endpoint (e.g. look up a company and then find credentials for that company)