Fix CONTRIBUTING.md: wrong repo references and tech prerequisites

[scubaninja]

Problem

CONTRIBUTING.md has two critical issues:

1. Wrong repository URLs (lines 3-4): Links point to github/startups-content instead of this repo
2. Wrong tech prerequisites (lines 17-18): Lists Go and golangci-lint, but this project uses Python (Flask/SQLAlchemy) and Node.js (Astro/Tailwind)

Fix

• Update fork/PR links to github-samples/pets-workshop
• Replace Go/golangci-lint prereqs with Python, Node.js, and git