[DOCS] Explain the possibility to augment the instrumentations offered by auto instrumentation

[lucabelluccini]

In the documentation page https://www.elastic.co/guide/en/apm/agent/dotnet/current/setup-auto-instrumentation.html we mention what we support out of the box for the auto instrumentation.

We should also call out those can be augmented through our instrumentation nuget packages.

Is your feature request related to a problem? Please describe.

Expectations on what are the technologies which can be instrumented and observed via auto instrumentation.

Additional context

It would be also nice to have a "comparison" page about strategies (when it is preferable to use Profiler runtime instrumentation , NuGet Packages or both).