Add Python example for thread-safe Define using rdfslot_

[gayatripadalia]

Summary

This PR adds a Python tutorial demonstrating thread-safe patterns in ROOT RDataFrame using rdfslot_.
In C++, ROOT provides DefineSlot and RedefineSlot for thread-safe operations by exposing the slot index to user-defined callables. These APIs are currently not available in PyROOT.

This tutorial shows how to reproduce the same behavior in Python by explicitly forwarding rdfslot_, enabling safe and lock-free access to per-slot resources in multi-threaded workflows.

Motivation

Issue #20839 highlights the lack of Python examples for thread-safe operations similar to DefineSlot.

This contribution provides a practical Python example of slot-based computation and demonstrates safe usage of mutable state in multi-threaded RDataFrame workflows, reflecting the design and intent of DefineSlot using existing PyROOT features.

Related Issue

Closes #20839

[vepadulano]

Dear @gayatripadalia ,

Thanks for attempting to increase the coverage of our suite of tutorials! The need to have more thread-safe examples is understood. Nonetheless, this PR does not contribute enough in that direction. Your example does not use the rdfslot_ implicit column in any meaningful manner, and it does not include examples of DefineSlot with comments motivating why that operation would be needed.

[gayatripadalia]

Dear @vepadulano,
Thank you for your helpful feedback.
I’ve updated the PR to better demonstrate meaningful usage of rdfslot_, including clearer examples of per-slot resource handling and explanations motivating thread-safe patterns similar to DefineSlot.
I wanted to check if the current version now aligns with what you had in mind, or if you would suggest any further improvements, especially regarding the examples or level of detail.
Thanks again for your guidance.