Performance: enable ssgWorkerThreads / upgrade to 3.9

[Blargian]

Summary

Enable SSG worker threads, which is new in their docs since I last tried to upgrade Docusaurus.
See facebook/docusaurus#10556

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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Project	Deployment	Actions	Updated (UTC)
clickhouse-docs	Ready	Preview	Feb 3, 2026 0:42am

[dhtclk]

Committed a change to remove a webpack optimization function, with experimental_faster enabled, it enables the use of Rspack, which isn't compatable, thus styles would not load.