Add CONTRIBUTING.md for Student-Portal | GSSoC'25

[DivyaJain-DataAnalyst]

The Student-Portal repository currently lacks a CONTRIBUTING file. Under GSSoC'25 open source contribution, this issue aims to introduce contribution guidelines to help new contributors understand the process of setting up the project, coding standards, and submitting pull requests. This will enhance collaboration and maintain consistency across contributions.

[khusshii26]

please assignt this to me