Document MDC usage in user documentation

[RiccardoManzan]

Feature description

It would be nice to have something in the description of micronaut-core and micronaut-tracing about the usage of MDC.
It looks like this module is required in order to make the MDC be correctly propagated in child processes.
It has been pretty difficult to find why the MDC values were not being shown by logback.

See:

• Document how to add request data to the SLF4J MDC micronaut-core#7914 (comment)
• https://stackoverflow.com/a/75871157/12048220

[scprek]

I've updated some docs for logback specifically in the java OTEL instrumentation project since it's technically implemented outside of micronaut and not included by the framework (See #166)

https://github.com/open-telemetry/opentelemetry-java-instrumentation/tree/main/instrumentation/logback/logback-mdc-1.0/library

Main readme
https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/docs/logger-mdc-instrumentation.md