docs: remove references to nonexistent requirements-dev.txt

[Ayomipo18]

Summary

GETTING_STARTED.md, CONTRIBUTING.md, and PROJECT_STRUCTURE.md instruct contributors to run pip install -r requirements-dev.txt, but that file does not exist in the repo, so setup fails at that step. All development tools (pytest, pytest-cov, black, flake8) are already provided by requirements.txt, so the extra install step is removed. PROJECT_STRUCTURE.md's dev-setup snippet now installs requirements.txt before the editable install so the documented flow works end to end.

Related Issue

N/A — documentation fix found during setup.

Type of Change

• Documentation update

Key Changes

• Remove pip install -r requirements-dev.txt from GETTING_STARTED.md
• Remove the same broken step from CONTRIBUTING.md
• Fix the dev-setup snippet in PROJECT_STRUCTURE.md to install requirements.txt before the editable install

Testing

• Followed the documented setup flow end to end with the corrected instructions

Checklist

• Self-review completed
• Documentation updated where needed