docs(otel): document shared execution trace

[ayushiahjolia]

Issue #, if available:
#674

Description of changes:

• Final PR in the stack for reparenting durable OTel spans onto one shared execution trace.
• Updates the OTel package README to match the shipped behavior: the shared execution trace model (Workflow and Invocation on one trace, anchored to a propagated backend parent or a deterministic synthetic root), same-trace ambient parenting, the two plugin trace-structure diagrams (InvocationOtelPlugin vs ExecutionOtelPlugin), the once-per-invocation sampling precedence, and the ExtractedContext/Sampling extractor contract (custom extractors return ExtractedContext, not an OpenTelemetry Context).

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