Adding section that details API Endpoints by Component

[eric-schuh]

This PR looks to resolve https://github.com/w3c-ccg/vc-api/issues/285

A section has been added to the "The VC API " section of the document called "API Component Overview". This section summarizes all endpoints in the VC-API, grouping them by the component the endpoint is expected to live on, along with a table for each of these components that lists the API endpoint, as well as the component that is the expected caller of the endpoint.

The workflow/exchanges endpoints are not currently indexed but should be added after the following PR is completed: https://github.com/w3c-ccg/vc-api/pull/382

---

Preview | Diff

[eric-schuh]

From review on 5/14 call:

• Currently the way the auto generation is working the "server" parameter from the OAS3.0.0 specification, we want to shift to using an x-parameter
• What this has caused is that some endpoints like /credentials which exist on multiple components at this point have overloaded expected callers
• Do we want to add descriptions to these tables? Currently the endpoint descriptions live in the Issuing/Verifying/Presenting sections of the document

[eric-schuh]

Updated to use the new parameter x-expectedCaller in yml files, also added x-expectedCaller to the new exchanges.yml endpoints.