Documentation for Service SDK for .NET

**What content needs to be created or modified?**
Service SDK docs for .NET

**Describe the solution you'd like**
What integration capabilities exist and what the intended use cases are

**Where should the new material be placed?**
Developing applications / SDKs / .NET / Server / ASP.NET

**The associated pull request from dapr/dapr, dapr/components-contrib, or other Dapr code repos**
n/a

**Additional context**
Missing documentation for .NET Service SDK raises concerns about maturity and readiness of Dapr for production use.