Document Metadata Enrichment for Elastic Agent and Beats Scenarios

**Describe the doc enhancement:**
This issue includes the documentation enhancement needed to add information for the metadata enrichment for kubernetes integration workflow. The metadata enrichment is a complicated mechanism that is mainly done by enrichers since https://github.com/elastic/beats/pull/27689

The `add_resource_metadata` config block is shared in [elastic-agent-autodiscover](https://github.com/elastic/elastic-agent-autodiscover/blob/main/kubernetes/metadata/config.go#L59) library but it provides different result if it is used inside the kubernetes provider or if it used inside the metricset block

The add_kubernetes_metadata processor currently starts in the background of Elastic-Agent but not in beats if the configuration not specifically implies to.

The kubernetesprovider is responsible for the completion of dynamic variables but on is own does not enrich the kubenretes events.
To make things more complex, there are diffrences in the available options that someone can configure in managed vs standalone agent. Also there are differences between elastic agent and filebeat/ metricbeat configuration

**Describe a specific use case for the enhancement or feature:**
All the above make clear that specific examples for a) Managed Elastic Agent with K8s integration, b) Standalone Elastic Agent with K8s integration, c) Filebeat with K8 logs collection (with and without autodiscovery) and d) Metricbeat with K8s metrics collection should be clarified and documented for our end users

Tasks:

- [ ] Beats
- [x] Elastic Agent PR: https://github.com/elastic/ingest-docs/pull/990