[docs] create data mapping dictionary for otel -> ecs mappings

[simitt]

Make it easier for anyone to understand which otel semantic conventions are mapped to ECS fields when processing with apm-data logic. The challenge will be to keep this up to date when done manually.
The main audience for this documentation are UI devs, users, PMs & support engineers.

[graphaelli]

I think we need something user facing for this as well. I've had a few user conversations asking how to get the service map working with otel or how to populate the failed transactions chart, which are partially things we should include a best practices guide but could more immediately be captured in a mapping guide.

To be clear, for this issue we would document: span.SetStatus ⇒ event.outcome.
For a user facing guide, we could enhance the docs with this information and suggest users always set a status.