Kubernetes guides

We recommend deploying APM Server using ECK. This issue is for tracking alternatives, following up on some requests - #929, elastic/apm-server#1244 - and confusion - [apm-server as daemonset](https://discuss.elastic.co/t/apm-server-as-kubernetes-daemonset/161307), provide guidance for running:

- [ ] apm-server as a daemonet along the lines of [auditbeat](https://www.elastic.co/guide/en/beats/auditbeat/7.0/running-on-kubernetes.html), [filebeat](https://www.elastic.co/guide/en/beats/filebeat/7.0/running-on-kubernetes.html), and [metricbeat](https://www.elastic.co/guide/en/beats/metricbeat/7.0/running-on-kubernetes.html)
- [ ] apm-server as a deployment
- [ ] apm-server with helm
- [ ] application w/apm agent