Update kafka ClickPipe setup

[kschlarman]

Summary

• Adding VerticalStepper to make it easier to read
• Updated screenshots
• Sized all images to md instead of lg
• Removed a few screenshots & simplified some text
• Added section to configure RPEs (points to private link docs)
• Extracted and revised section on schema registries

https://clickhouse-docs-5nd9brjz7-clickhouse.vercel.app/docs/integrations/clickpipes/kafka#creating-your-first-kafka-clickpipe

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx

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[Blargian]

@kschlarman big improvement already I think! What do you think if I split it up into separate pages as:

• Create your first Kafka ClickPipe
• Schema registries
• Reference (we can group supported formats, sources, delivery semantics, authentication etc on this)
• Best practices
• FAQ

Each of these pages will then show as sub pages under "ClickPipes for Kafka" and we add a table of contents table on that page instead. I'll PR after we merge these changes in.

[kschlarman]

@Blargian Yep, that sounds great.

[genzgd]

Sorry I missed the Avro unions part previously. Otherwise see comments.