Backfill guide

[gingerwizard]

Summary

Guide on how to backfill data inc. mvs. cc @tom-clickhouse

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/en/integrations/index.mdx

[tom-clickhouse]

slight logic issue