Document that pipeline needs to be lowercase

[belimawr]

Proposed commit message

Document that the ingest pipeline ID from Elasticsearch defined in the input or output configuration is always lowercased, thus the ingest pipeline in Elasticsearch can only use lowercase names.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

## Disruptive User Impact
## Author's Checklist

How to test this PR locally

1. Clone the docs repository
2. Make sure beats and docs have the same parent folder
3. Define $GIT_HOME as the parent folder from beats and docs
4. Build the docs, a browser window will open with the rendered docs

$GIT_HOME/docs/build_docs --respect_edit_url_overrides --doc $GIT_HOME/beats/filebeat/docs/index.asciidoc --resource=$GIT_HOME/beats/x-pack/filebeat/docs --chunk 1 --open

5. Validate the changes in http://localhost:8000/guide/elasticsearch-output.html and some inputs like http://localhost:8000/guide/filebeat-input-filestream.html#_pipeline_9

Related issues

• Closes Document that when using pipeline the name is always lowercased #41224

## Use cases
## Screenshots
## Logs

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.