Add migration guide docs

[stevejgordon]

We should have a migration section in the docs which covers migrating two scenarios:

• Migrate from Elastic APM Agent (ASP.NET Core)
• Migrate for "vanilla" OpenTelemetry SDK (ASP.NET Core) where the before code adds OTLP + instrumentation. Show that we can reduce the code needed for this common scenario.

[estolfo]

This blog post about the Go agent might be a useful reference.