docs: vendor ClickHouse documentation

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Summary

Make ClickHouse/dbt-clickhouse the source of truth for its public ClickHouse documentation.

• Copy the current dbt documentation from ClickHouse/ClickHouse into docs, including repository-owned navigation.
• Verify the complete documentation slice against a pinned ClickHouse docs snapshot with the scoped Mintlify and internal-link checks.
• Open or refresh an automated synchronization pull request in ClickHouse/ClickHouse after releases, manual dispatches, or merged pull requests carrying the sync-docs label.
• Point contributor guidance and the pull request checklist at the repository-owned documentation.

The published documentation URLs remain unchanged. Package behavior is unaffected, so no changelog entry is needed.

Related: ClickHouse/ClickHouse#116357
Related: ClickHouse/ClickHouse#116726

This draft depends on a companion ClickHouse/ClickHouse change that replaces the inline dbt sidebar definition with a reference to data-ingestion/etl-tools/dbt/navigation.json. After that change merges, CLICKHOUSE_DOCS_REF must be updated to its merge commit before this pull request is merged.

Validation: actionlint, yamllint, scoped Mintlify validation, navigation coverage, and 84,162 internal link and anchor checks passed with zero errors.

Checklist

• For significant changes, documentation in docs was updated with further explanations or tutorials

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How it works:

• It adds a workflow which shallow clones the full docs site and overlays the docs in this repo to check that they build correctly, that there are no broken links etc.
• Docs updates can be made in this repo and when a new release is created, a PR will automatically open on ClickHouse/ClickHouse to update the docs
• If you need to sync docs before the release you can add the sync-docs label to the PR and it will open a sync PR with the changes on ClickHouse/ClickHouse on merge of the merge