docs: improve contributor setup and PR template

[sahilmathur254]

Summary

• Expand CONTRIBUTING.md with prerequisites, fork and local setup steps, validation commands, and pull request guidance.
• Update the existing pull request template with the requested change-type, testing, and checklist sections.
• Keep the canonical lowercase template path required by the repository charter.

Closes #16.

Type of change

• Documentation
• Bug fix
• New or updated learning content
• Repository tooling
• Other

How Has This Been Tested?

• uv run scripts/validate_all_modules.py
• uv run scripts/validate_ci_gate.py
• uv run scripts/run_topic_tests.py TARGET
• Manual validation (details below)

Validation ran in an isolated Python 3.12 Docker environment:

• python -m mdformat --check CONTRIBUTING.md .github/pull_request_template.md
• python scripts/validate_ci_gate.py passed; 15 of 16 modules validated and the gate accepted the repository's existing transitional FastAPI exception.
• python scripts/run_topic_tests.py 01_python_fundamentals/advanced_strings passed all 5 tests.
• git diff --check passed.

Checklist

• I followed the contribution guidelines.
• I kept the change focused and used English throughout.
• I added or updated tests and documentation where applicable.
• I did not introduce TODO placeholders or unrelated changes.

[ulises-jeremias]

Thanks @sahilmathur254! Looks good on a quick pass. CI has not reported checks yet on this branch — once workflows run green we can merge.