fix(docs): change npm install to pnpm install in README and ignore package-lock.json (#2694)

[desireddymohithreddy0925]

Summary

Resolves the conflicting package manager instructions by updating the README to recommend pnpm instead of npm.

Closes #2694

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• 📝 Documentation update

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What Changed

• Replaced npm install with pnpm install in the Quick Start guide within README.md.
• Added package-lock.json to .gitignore to prevent npm lockfile pollution.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus