docs(cursor): add tutorials for Cursor (Signup, IntelliJ, VS Code)

[arshinsikka]

Summary

Added three new tutorials under AI-Assisted Coding for using Cursor.

Details

• cursorSignup.md: Step-by-step guide to creating a Cursor account and logging in.
• cursorIntellij.md: Guide to using Cursor with IntelliJ IDEA via chat and refactor/explain workflows.
• cursorVscode.md: Detailed walkthrough for Cursor in VS Code, including inline completions, chat, inline chat, refactoring, and responsible usage.

Motivation

This fills a gap in AI-assisted coding tutorials by covering Cursor, which is becoming increasingly popular among students.
It complements the existing Copilot and Windsurf tutorials, while highlighting Cursor as a practical "ChatGPT inside your IDE."

Notes

• Student plan details (1-year free Pro in some regions) are included in the signup guide.
• Screenshots can be added later to enhance the tutorials.

Closes #170

[damithc]

@arshinsikka
Images are missing in the PR.
Follow the file name style used by other VS Code and Intellij tutorials.
index.md needs to be updated to point to these tutorials.