[Internal]: Add or reference Beats docs in Elastic Agent docs

### Description

Users need to browse to the Beats docs to find explanation on how to set up certain aspects of the Elastic Agent. For instance, data duplication and strategies to prevent it [is documented here](https://www.elastic.co/docs/reference/beats/filebeat/filebeat-deduplication), but cannot be found in the Elastic Agent documentation.

It would make sense to cross-reference the docs, as we do mention options in [this blog post for example](https://www.elastic.co/blog/log-deduplication-with-elasticsearch).

### Resources

Blog: https://www.elastic.co/blog/log-deduplication-with-elasticsearch
For example on what we actually document on the agent / Fleet side are processors: https://www.elastic.co/docs/reference/fleet/agent-processors
Beats documentation: https://www.elastic.co/docs/reference/beats/filebeat/filebeat-deduplication

### Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

This is about configuring Elastic Agent and cross-referencing Beats documentation, so affects all platforms.

### What release is this request related to?

N/A

### Serverless release

N/A

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** @emrcbrn

**Stakeholders:** Unknown