Fix top level menu links which go to the wrong place

[Blargian]

Summary

Fixes links for 'Migrating data' and 'Data sources' which are linked to pages which no longer exist.

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]

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