Fix tracing docs for ASP.NET Core, WebAssembly etc.

[jamescrosswell]

Description

https://docs.sentry.io/platforms/dotnet/guides/aspnetcore/tracing/trace-propagation/
https://docs.sentry.io/platforms/dotnet/guides/blazor-webassembly/tracing/trace-propagation/#how-to-use-distributed-tracing
These are the wrong docs ☝🏻

They explain how to configure tracing for old aspnet framework apps.

There also isn't much on SentryMessageHandler. There's a note in https://docs.sentry.io/platforms/dotnet/configuration/http-client-errors/ about it but we could make something more prominent.

[linear]

NET-577