docs(kafka): Schema registry documentation

[johnBgood]

Description

Feature Request: support for schema registry in the Kafka connector. It will be released with 8.6 (not backported).

Related issues:

• https://github.com/camunda/team-connectors/issues/874
• https://github.com/camunda/team-connectors/issues/646

When should this change go live?

• [ ] This is a bug fix, security concern, or something that needs urgent release support.
• [ ] This is already available but undocumented and should be released within a week.
• [ ] This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• [x] This is part of a scheduled alpha or minor. (apply alpha or minor label)
• [ ] There is no urgency with this change and can be released at any time.

PR Checklist

• [ ] My changes are for an already released minor and are in /versioned_docs directory.
• [ ] My changes are for the next minor and are in /docs directory (aka /next/).

• [ ] My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• [x] My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.