Add annotation docs for java instrumentation

[makeavish]

Java otel agent supports adding custom spans without too much change: https://opentelemetry.io/docs/instrumentation/java/automatic/annotations/

It is different from manual instrumentation which is tedious and requires too much code changes. I have tested annotations with sample java app and it works fine without any extra configurations.

[Calm-Rock]

@makeavish do we have docs for this ?