APM mappings changes should be shown in release notes, improve guidance on customizations on the documentation

[lucabelluccini]

This issue is to track 2 possible coupled items:

• Whenever we change any field type in the built-in APM assets, it should show up in the release notes as breaking change
• In the @custom doc section of our O11y docs, we should recommend to not override the fields we declare but prefer moving the fields to a different "namespace" (e.g. custom.myfield as it's less likely this will trigger a mapping conflict). In order to do so, the user would need to introduce an ingest pipeline which extracts the relevant fields and moves / processes them and finally stores them into custom.<fieldname>, plus add a mapping to the @custom` component template (example below)

In short, if a customer wants to extract a specific HTTP header, instead of overriding http.request.headers.myheader mappings, it would be better to:

• define a traces-apm@custom ingest pipeline where we take http.request.headers.myheader and store it on custom.myheader
• define a traces-apm@custom component template to specify the mapping of custom.myheader to whatever type the user wants