refactor Postgres source slightly, link MySQL ClickPipe

[heavycrystal]

Summary

clean up Postgres data source a bit
add a call to action to MySQL data source linking to MySQL 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