[REQUEST]: Missing APM Agents setup for Kubernetes env.

### Description

In this document: https://www.elastic.co/guide/en/observability/master/monitor-kubernetes.html#_step_3_install_and_configure_apm_agents

It is using `init container` and setup [volumeMounts](https://github.com/elastic/observability-docs/blame/917c947fe10fa0b65ef380f767b7a55fd6216548/docs/en/shared/install-apm-agents-kube/java.asciidoc?plain=1#LL22C12-L22C12) as below:

```
volumeMounts:
- mountPath: /elastic/apm/agent
name: elastic-apm-agent
```

This should be included in the containers setup, too.
I would like to add the 3 lines above under [this line](https://github.com/elastic/observability-docs/blame/917c947fe10fa0b65ef380f767b7a55fd6216548/docs/en/shared/install-apm-agents-kube/java.asciidoc?plain=1#L43).

Also need to add the explanation too in the contents above the code snippet. It should be aligned with the added code.

### Resources

https://www.elastic.co/guide/en/observability/master/monitor-kubernetes.html#_step_3_install_and_configure_apm_agents

### Collaboration

The documentation team will investigate the issue and create the initial content.

### Point of contact.

**Main contact:** @insukcho

**Stakeholders:**