ADP changes for Voice API spec update (CAPI endpoints)

[abdulajet]

Duped the CAPI docs (code snippets, concepts and use case) for voice API https://nexmo-developer-pr-3865.herokuapp.com/voice/voice-api/code-snippets/conversation/create-event https://nexmo-developer-pr-3865.herokuapp.com/use-cases/retrieve-conversation-details-voice Combined the VAPI call and legs guides (https://nexmo-developer-pr-3865.herokuapp.com/voice/voice-api/concepts/callandleg) Exposed the Client SDK Voice tutorials to the VAPI menu https://nexmo-developer-pr-3865.herokuapp.com/voice/voice-api/tutorials/app-to-phone/introduction/javascript