Document how to build and run APM Server in its various modes (dev docs)

We should provide developer docs explaining how to build APM Server in each of the ways it can be run, for development/testing purposes. Particularly:
- how to build a standalone apm-server Docker image
- how to build a derivative Elastic Agent Docker image with a local build of apm-server injected, and how to run it in Elastic Cloud

At the same time, document how to build a derivative Kibana Docker image with a local build of the APM integration package injected, and how to run it in Elastic Cloud.