feat(revision): add Day 03 - Stacks & Queues (Valid Parentheses, Next Greater Element)

[DeveloperViraj]

Added new "Day 03 - Stacks & Queues" folder under CPP/algorithms/revision/day-03-stacks-queues/.

Included two problems:

1. Valid Parentheses
2. Next Greater Element

Both implemented using stack-based approaches with clear algorithm descriptions, complexity notes, and sample I/O.

[github-actions]

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[DeveloperViraj]

@Karanjot786 @Pradeepsingh61
All checks have passed, and both files are correctly placed under CPP/algorithms/revision/day-03-stacks-queues/.
Kindly review and merge when convenient 🙌
Thanks again for maintaining this awesome repo 🚀