feat: implement optimized two-pointer approach for trapping rainwater (#2975)

[kanavgoyal898]

Description of Change

Refactored the implementation of the Trapped Rainwater problem to use the two-pointer approach, reducing the space complexity from O(n) to O(1) while maintaining O(n) time complexity. This improves efficiency and aligns with optimal algorithmic practices. The Doxygen-style documentation and metadata were also updated to reflect the change in logic and complexity.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
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Notes: Optimized the Trapped Rainwater solution using a two-pointer technique to improve space efficiency without sacrificing readability or correctness. Suitable for competitive programming and educational references. Fixes #2975