Added: TOA validation notebook tested with latest dependencies (GSoC warm-up)

[Anithpavan]

This PR adds the validate_toa_calculation.ipynb notebook under the validation/ directory.

• Verified execution using the latest stingray and Python environment.
• Added markdown comments explaining the outputs and validation notes.
• This supports GSoC warm-up and refers to issue #802.

Looking forward to feedback. Thanks!

[Anithpavan]

Hi! 👋 This is part of my GSoC warm-up task. Kindly requesting a review when you get a chance 😊

[ankitkhushwaha]

Hey @Anithpavan , u may have pushed the wrong branch. Check it, it is the same pr that was closed earlier.

[matteobachetti]

@Anithpavan If there was no change to the code, I will close this. Thank you for checking!