Document the need to mark libraries that should be otel instrumented as external when using bundling solutions like AWS SAM for AWS Lambda

### Description

Using frameworks like AWS SAM or AWS CDK to upload and maintain AWS Lambda functions usually bundles all of the code and its dependencies into a single file.

This prevents otel from hooking into any libraries it [offers custom instrumentation](https://github.com/open-telemetry/opentelemetry-js-contrib/tree/main/plugins/node) for, e.g.`pg`.

### Solution Brainstorming

Document the need to mark libraries that should be instrumented as external and the need of a lambda layer to hold such dependencies when bundling AWS Lambda functions.

Once/if https://github.com/open-telemetry/opentelemetry-js/issues/4174 lands, we can consider removing this documentation again.