Relocate R tutorial to machine learning section and make it language-agnostic

[coderabbitai]

Description

The R tutorial document currently located at docs/integrate/r/tutorial.md should be relocated to the machine learning section (docs/solution/machine-learning/) and refactored to be language-agnostic.

Current location

• https://cratedb.com/docs/guide/integrate/r/tutorial.html
• File: docs/integrate/r/tutorial.md

Target location

• https://cratedb.com/docs/guide/solution/machine-learning/
• Directory: docs/solution/machine-learning/

What needs to be done

1. Move the tutorial from docs/integrate/r/ to docs/solution/machine-learning/
2. Refactor the content to be language-agnostic (not R-specific)
3. Update cross-references and navigation
4. Ensure the tutorial fits well within the machine learning section structure

Context

The tutorial covers the iris classification problem using R and CrateDB. The content should be generalized to focus on the machine learning concepts and CrateDB integration rather than being R-specific.

References

• Related PR: Driver: Add page about R #405
• Comment: Driver: Add page about R #405 (comment)
• Requested by: @amotl