Add Incremental Queries documentation

[va3093]

Summary

• Add new documentation page for the Incremental Queries feature under Query Engine > Functionality
• Explains the TABLE(previous.result(...)) table function pattern for referencing previous query results
• Includes mermaid diagrams for execution flow and data flow visualization
• Provides 3 complete example queries demonstrating common patterns
• Compares Incremental Queries vs dbt Connector, positioning dbt as the full-featured solution
• Updates "Writing Efficient Queries" page with reference to new documentation

[cursor]

PR Summary

Adds comprehensive docs for incremental processing in Dune.

• New query-engine/incremental-queries page explaining TABLE(previous.result(...)), the standard incremental pattern, mermaid diagrams, 3 end-to-end SQL examples, tips, and a comparison with the dbt Connector
• Updates docs.json to surface Incremental Queries under Query Engine > Functionality
• Enhances writing-efficient-queries.mdx with a new section linking to Incremental Queries for recurring workloads (plus minor formatting cleanup)

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