Extend the topic description for the Kafka output

[vishaangelova]

This PR extends the Topics settings section of the Kafka output docs for both standalone and Fleet-managed Elastic Agents, and:

• adds missing information about setting dynamic topics in the Kafka output doc for standalone Elastic Agents,
• adds additional information and an example for using the add_fields processor for setting complex, dynamic topic value in both docs,
• fixes a typo and a link text.

Resolves elastic/ingest-docs#1780

Preview

reference/fleet/kafka-output-settings.md
reference/fleet/kafka-output.md

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/kafka-output-settings.md
• reference/fleet/kafka-output.md

[vishaangelova]

@kpollich in the issue, you mentioned

The proposed workflow here would be to add the block above to any integration policies on Fleet-managed agents or to the input config on any standalone agents that output to Kafka

So I made an assumption that dynamic values can also be set for topic in the Kafka output configuration for standalone agents. Since that information was not in the doc, I copied it from the doc for Fleet-managed agents. I hope I’m not wrong...

[benironside]

LGTM, left one minor suggestion

[kpollich]

LGTM, thank you! 🚀