clickpipes: misc improvements to ClickPipes for Kafka documentation

[morsapaes]

Summary

• Improve visibility for Azure Event Hubs FAQ (reported on Slack by @cwurm). In the future, we should add dedicated guides per supported service that highlight service-specific nuances (like we have for CDC ClickPipes).
• Update the link to ClickPipes pricing (reported on Slack by @kschlarman). Relatedly, the January pricing update is being removed in clickhouse-docs-private #169.
• Remove references to Upstash, which has been sunsetted.

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

[vercel]

@morsapaes is attempting to deploy a commit to the ClickHouse Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Comments	Updated (UTC)
clickhouse-docs	Ready	Preview		Aug 26, 2025 4:27pm

2 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
clickhouse-docs-ru	Ignored	Preview		Aug 26, 2025 4:27pm
clickhouse-docs-zh	Ignored	Preview		Aug 26, 2025 4:27pm