Split Key Concepts into Key Concepts and Trinsic Platform; rename Services to Reference

[geel9]

This PR splits up the Learn -> Key Concepts section into two sections: Key Concepts and Trinsic Platform

• Key Concepts should introduce readers to the concepts behind Identity/SSI in general
• Trinsic Platform should document the intricacies of working with Trinsic specifically.
• This is just an organization change; the contents of these documents should likely change to better fit this categorization.

I can see some overlap between these two -- for instance, if we talk about credential schemas in a document in Key Concepts, we could link to the Trinsic Platform -> Templates document, IE, "here's how we handle this concept"

Additionally, this PR changes the Services tab to Reference -- it was named "services" to refer to the internal services that the SDK exposes; however, it communicated more of a business topic ("the services Trinsic offers") than what it actually was: an API reference.

Partially handles #748

[tmarkovski]

I agree with this breakdown