Added details to trouble shoot launch of the gRPC server under Nginx on Linux

After few hours of struggle and many reading materials - I realized that Nginx does not support HTTP/2 to upstream server and therefore was getting all the time HTTP/1.x protocol mismatch exceptions.

It turned out that you have proxy to either `grpc://...` or `grpcs://...` (secure Kestrel) upstream server. Support for gRPC has been added in Nginx v1.13.10.

Would be great to note this down somewhere (probably here as it applies specifically to host gRPC under HTTP/2) to save time for others.

Thanks!

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#### Document Details

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* ID: c730b5f9-031e-beab-b838-6e2f8befde30
* Version Independent ID: 84ddcb67-03be-655c-470a-f43a5152606f
* Content: [Troubleshoot gRPC on .NET Core](https://docs.microsoft.com/en-us/aspnet/core/grpc/troubleshoot?view=aspnetcore-3.1)
* Content Source: [aspnetcore/grpc/troubleshoot.md](https://github.com/dotnet/AspNetCore.Docs/blob/master/aspnetcore/grpc/troubleshoot.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-grpc**
* GitHub Login: @JamesNK
* Microsoft Alias: **jamesnk**