Docs: Extend the topic description in the Kafka output doc

[vishaangelova]

Docs

This PR extends the topic description in the Configure the Kafka output doc for Filebeat to include information about setting dynamic topic values.

Resolves #42932

Preview

https://docs-v3-preview.elastic.dev/elastic/beats/pull/46909/reference/filebeat/kafka-output#topic-option-kafka

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.

[github-actions]

🤖 GitHub comments

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @vishaangelova? 🙏.
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
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/filebeat/kafka-output.md

[vishaangelova]

@pierrehilbert, could you direct me to someone from your team who can review this PR? If the proposed changes for the Kafka output are valid for Filebeat, would the same information also apply for the rest of the beats? (If yes, I’d also like to update the Kafka output docs for the other beats in this PR.)

[pierrehilbert]

Hey @vishaangelova,

@faec would be the best to review this.