Fix #607: Input validation in Number Guessing App

[bhandareasmita]

Type of change

• Added new project
• [ ✔] Bug fix
• [ ✔] New features
• [ ✔] Enhanced documentation

Changes proposed in this pull request: Enhanced features like input validation and docstrings #607

• Project name : Number Guessing App

• Short description : Added user validation for two game modes with docstrings for respective functions

Checklist

• [✔ ] The code is Python 3 compatible
• [✔ ] I have added relevant comments in my file to make code understandable
• I have added my project in a folder which includes all the necessary files

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