docs: add question about the purpose of the super keyword in Java

[vardhan30016]

What I Added

• Added a new question: "What is the purpose of the super keyword in Java"
• Included it in the Table of Contents.
• Explained the main uses of the super keyword:
  • Accessing parent class variables.
  • Calling parent class methods.
  • Invoking parent class constructors.
• Added a sample code example and output in the same format as existing questions.

Why This Change

The super keyword is a fundamental concept in Java inheritance and is frequently asked in interviews.
Adding this question improves the collection of beginner-friendly Java interview questions.

Additional Notes

• Follows the existing markdown structure and formatting.
• Verified code example and syntax.

[sudheerj]

@vardhan30016 Looks like the conflict still exists. Please check it once.