Add parsers documentation to Journald docs

[belimawr]

Proposed commit message

See title

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

Build Filebeat docs:

1. Clone Beats and Docs
2. Checkout this PR/Branch on Beats
3. Build the docs:

   export GIT_HOME=<path to the folder where Beats and Docs have been cloned>
   $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
   

4. Open http://localhost:8000/guide/filebeat-input-journald.html
5. Validate the parsers documentation.

Related issues

• Closes [Journald] Document the parsers #40478

## Use cases
## Screenshots
## Logs

[elasticmachine]

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

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @belimawr? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[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.

Shouldn't have approved.