Add SQL-to-Kotlin DataFrame transition guide for backend developers

[zaleslaw]

No description provided.

[Copilot]

Pull Request Overview

This PR adds a comprehensive guide for backend developers with SQL experience to transition to Kotlin DataFrame. The guide provides SQL-to-DataFrame mappings and explains key conceptual differences between SQL databases, ORMs, and Kotlin DataFrame.

• Introduces Kotlin DataFrame concepts through familiar SQL terminology
• Provides side-by-side comparisons of SQL commands and DataFrame operations
• Explains the differences between DataFrame, SQL databases, and ORMs

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Guide-for-backend-SQL-developers.md	New comprehensive guide document with SQL-to-DataFrame mappings and conceptual explanations
d.tree	Adds the new guide to the documentation navigation structure

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[AndreiKingsley]

Please add an information about supported SQL DBs (and how to integrate unsupported !) with references to https://kotlin.github.io/dataframe/sql.html.

[koperagen]

Can be interesting that columns store any values, not just primitives or predefined set of types. List, File, Map, user objects - DataFrame offers fully generic storage.
(It's mentioned in It can be created from any source but i missed it, my attention went mostly to tables :) )