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

## 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: #405
- Comment: https://github.com/crate/cratedb-guide/pull/405#discussion_r2449677582
- Requested by: @amotl