Document io.micrometer:context-propagation

[wetted]

closes #384

These are the docs from https://docs.micronaut.io/latest/guide/#reactorContextPropagation

@dstepanov do we want to add something about ReactorAutomaticContextPropagation in this module?

@sdelamo I think we need an issue for core to remove the docs there and add a link to these docs, yes?

[dstepanov]

We might want to add here or, better core, a note on how to add the context in the middle of the reactive chain:

PropagatedContext propagatedContext = PropagatedContext.get();
    mono.contextWrite(ctx -> ReactorPropagation.addPropagatedContext(ctx, propagatedContext))

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