Add storage policy documentation

[glasstiger]

Summary

• Add new Storage Policy concept page (concepts/storage-policy.md) documenting the Enterprise feature for automated partition lifecycle management (convert to Parquet, drop native, drop local, drop remote)
• Add new ALTER TABLE SET STORAGE POLICY SQL reference page (query/sql/alter-table-set-storage-policy.md) with full syntax, constraints, permissions, and examples
• Add STORAGE POLICY clause documentation to the CREATE TABLE reference page
• Update data-retention.md to list storage policies as a third retention approach alongside TTL and manual DROP PARTITION
• Update storage-engine.md to reference storage policies for automatic Parquet conversion (replacing the previous object storage mention)
• Add cross-reference from TTL concept page to the new storage policy docs
• Register both new pages in sidebars.js

Test plan

• Verify yarn build succeeds with no broken links
• Check sidebar navigation includes the new pages under Concepts and SQL Reference
• Confirm cross-links between storage policy, TTL, data retention, and CREATE TABLE pages resolve correctly
• Generate railroad diagrams for the new SQL commands (SET STORAGE POLICY, DROP STORAGE POLICY, ENABLE STORAGE POLICY, DISABLE STORAGE POLICY)

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[github-actions]

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