docs: describe how OTel resource attributes are translated and stored in labels

[carsonip]

There are no docs describing how only a subset of OTel resource attributes are translated to ECS, and how the rest are converted, e.g. replacing . with _, and stored under labels.*. These pages are good candidates to describe this limitation.

• https://www.elastic.co/guide/en/observability/current/apm-open-telemetry-resource-attributes.html
• https://www.elastic.co/guide/en/observability/current/apm-open-telemetry-known-limitations.html