Add documentation about when to use Corvus.Monitoring vs System.Diagnostics

Since Corvus.Monitoring was first built, things have moved forward in the framework, as it continues to align with OpenTelemetry.

It's possible to use the `ActivitySource` and `Activity` classes in the `System.Diagnostics` namespace, together with the latest OpenTelemetry library to achieve a similar effect to Corvus.Monitoring, and this is likely the correct approach for greenfield apps.

There are some differences - for example, Corvus.Monitoring allows adding metrics to an operation, which doesn't appear to have an equivalent with OTel, and `ActivitySource` allows you to specify the kind of trace you'd like to create when you start a new activity. However, broadly speaking, the two approaches are very closely aligned.

We may also want to consider adding a System.Diagnostics based implementation of the Corvus.Monitoring abstractions.