Document config knobs for .NET Core

[gewarren]

Document supported run-time configuration knobs for .NET Core apps. Include runtimeconfig.json and environment variables for each, as well as the corresponding .NET Framework setting (app.config). Also include the first version it was available in.

Reference:

• https://github.com/dotnet/coreclr/blob/ed5dc831b09a0bfed76ddad684008bebc86ab2f0/src/vm/eeconfig.cpp
• Document new GC configuration knobs coreclr#25657
• https://github.com/dotnet/coreclr/blob/master/src/inc/clrconfigvalues.h
• https://devblogs.microsoft.com/dotnet/the-history-of-the-gc-configs/

Related to #13425.