docs(contributing): adds upstream setup and sync steps to Getting Started workflow

[Trinath052004]

Description

Updates the Getting Started guide to include steps for adding the upstream remote and syncing the local main branch with upstream before creating a feature branch.

This ensures contributors follow the correct fork-based workflow and helps prevent unnecessary merge conflicts.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #2692

Changes Made

• Added upstream remote setup step in Getting Started
• Added explicit instructions for syncing main with upstream before branching

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes