Unify tracing documentation

[jkotas]

The tracing documentation is spread over multiple places:

• https://docs.microsoft.com/en-us/dotnet/core/diagnostics/dotnet-trace
• https://github.com/dotnet/diagnostics/blob/master/documentation/dotnet-trace-instructions.md
• https://github.com/dotnet/runtime/blob/master/docs/project/linux-performance-tracing.md

We should have one copy documentation that is complete and correct - ideally, it should be the official docs site; and delete the other copies.

[tommcdon]

Closing as we are tracking documentation unification under #1682