[DOCS] Updating MNIST Tutorial

[Smartmind12]

What would you like to share?

The MNIST: Digits Classification Tutorial page currently only consist of codes to be run but does not mention what each line of code or function means nor their significance in the Model.

Suggestion: I would suggest updating the tutorials page so that it can act as a step by step guide for new learners and help them understand use of each and every function and argument applied in the ML Code.

Additional information

October-2022(Hacktoberfest)

[Smartmind12]

#self-assign

[Smartmind12]

@cosmicBboy @eapolinario Could you approve this issue so that i can submit the PR to it!