Document SSL/TLS key and certificate formats accepted by plugins

Plugins are not being specific about the accepted SSL key/certificate format. The manticore-based plugins, for example, are not documenting that the SSL keys should be in the `PCK8` format, otherwise it won't work.

A few examples where the documentation is not clear about that:

- https://www.elastic.co/guide/en/logstash/current/plugins-outputs-elasticsearch.html#plugins-outputs-elasticsearch-ssl_key
- https://www.elastic.co/guide/en/logstash/current/plugins-filters-elasticsearch.html#plugins-filters-elasticsearch-ssl_key
- https://www.elastic.co/guide/en/logstash/current/configuring-centralized-pipelines.html (`xpack.management.elasticsearch.ssl.key`)

The goal of issue is to review all plugins, and ensure that the ones supporting SSL/TLS configurations, have the key and certificate format specified on the docs.

---

```[tasklist]
### Tasks
- [x] Elasticsearch output (https://github.com/logstash-plugins/logstash-output-elasticsearch/pull/1181)
- [ ] ...
```

Start by reviewing the Elasticsearch output and filter documentation links, the centralized pipelines page, and the completed Elasticsearch output task. Then review the remaining plugins that support SSL/TLS configurations and document the accepted key and certificate formats; done means each relevant plugin's documentation specifies those formats.

Written by the indexing model from the issue text.