Document how to use Java agent with declarative config

[zeitlinger]

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

We have https://opentelemetry.io/docs/languages/java/configuration/#declarative-configuration

Describe the solution you'd like

We want an easy to use section under https://opentelemetry.io/docs/zero-code/java/agent/ with a step-by-step guide, including a ready-to-copy config that is good for most users.

Describe alternatives you've considered

No response

Additional context

No response