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

[andreiborza]

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 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 open-telemetry/opentelemetry-js#4174 lands, we can consider removing this documentation again.