docs: improve Windows & WSL clarity for Java + MongoDB quickstart

[SANSKRITI-2004]

Summary

• Enhanced the Quickstart guide for Spring Boot + MongoDB by:
  • Clarifying Windows vs WSL command usage
  • Adding a troubleshooting section
  • Improving formatting and subheadings

Why

These edits make the guide more beginner-friendly, especially for Windows users who face command issues. It adds clarity and helpful suggestions.