Added overview to dial-in page

docs/Telemetry/Dial-In/1-index.md

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 # Dial-In gNMI Overview
 
+## Dial-In Overview
+Dial-in telemetry uses model-driven methods to collect telemetry from a Cisco Nexus device. The term "dial-in" means the external collector is "dialing-in" to the Nexus device and telling the switch what to do. This process happens in two steps. The first step is for the external collector to connect to the Nexus device and tell the switch which telemetry data to collect. The second step is for the Nexus device to collect this data and send it to the external collector.
+
+Dial-in telemetry has multiple methods available for the data structure, encoding, and transport.
+
 ## About gRPC
 Google Remote Procedure Call (gRPC) is an open source high performance RPC framework released in 2016. gRPC has many RPCs available to control systems. It uses HTTP/2 for transport, protocol buffers for the interface description language, and includes the following features: