gRPC documentation and wizard

The gRPC code came in as a contribution, we merged and released.
We haven't added any docs for this yet.

## Docs

There are [docs on how to contribute to the docs](). :)
But basically this is like the [ASP.NET Core docs]() and [goes under `platforms/dotnet/guides`]().

## Wizard

The wizard is a single file named after the platform (here it could be `aspnetcore-grpc.md` and goes in this folder:

[https://github.com/getsentry/sentry-docs/tree/master/src/wizard/dotnet]()