document apm package bundling with Fleet/Kibana

The question how the apm package is bundled with Fleet/Kibana and why there are no more releases of production versions of apm package available at the package registry is a continuous source of confusion.

We should document
- the flow how the bundled apm package gets updated for internal developers
- that the apm package is not expected to be available at the package registry in a production version as it is bundled with Kibana.

Trace how the apm package is bundled with Fleet/Kibana and how bundled updates reach internal developers. Document that flow and clarify why production versions are not expected at the package registry because the package is bundled with Kibana; the issue does not name specific files or tests to begin with.

Written by the indexing model from the issue text.