Reduce duplication in documentation

Since we migrated to markdown docs in 9.0, each Beat has their own set of documentation files, even for the shared components like processors. This generates a lot of duplication and toil when any shared component needs to have their documentation updated.

Here is an example for the add docker metadata processor:
```
% find . -name 'add-docker-metadata.md'
./docs/reference/auditbeat/add-docker-metadata.md
./docs/reference/heartbeat/add-docker-metadata.md
./docs/reference/filebeat/add-docker-metadata.md
./docs/reference/winlogbeat/add-docker-metadata.md
./docs/reference/packetbeat/add-docker-metadata.md
./docs/reference/metricbeat/add-docker-metadata.md
```

Some of those files are also pretty much duplicated in the Fleet documentation that resides at [docs-content](https://github.com/elastic/docs-content), ex: https://github.com/elastic/docs-content/blob/main/reference/fleet/add_docker_metadata-processor.md

Ideally we would have a single source of truth [like in 8.19](https://github.com/elastic/beats/blob/8.19/libbeat/processors/add_docker_metadata/docs/add_docker_metadata.asciidoc) that gets rendered for each Beat during the docs build process.

This issue is about discussing the possibility of having some automation that can render the duplicated docs from a single source of truth. The processors are a good candidate to start with, but there are many sections in the Beats documentation that are the same for all Beats.

Compare the duplicated processor pages under docs/reference/*/add-docker-metadata.md with the historical source at libbeat/processors/add_docker_metadata/docs/add_docker_metadata.asciidoc and the Fleet copy in docs-content. Start by mapping which sections are shared and how the docs build currently renders them. Done means a concrete automation design or implementation path that supports a single source of truth across Beats documentation.

Written by the indexing model from the issue text.