Add pull request template for improved contribution guidelines

[RafsanNeloy]

Added general contributor checklist and quality-gate docs which was outlined as task 13 of Phase 2 from plan.md.

[dlq]

This is a useful addition and the structure is clear. The branch/test/security checklist plus the test-plan section should make future reviews easier!

One small follow-up before we merge: since this is covering plan.md item #13, could you add the remaining CBRAIN-specific quality gates from that item too? In particular, please mention checking JSON/JSONL output modes when command behavior changes, avoiding direct printing from data-layer code, preserving public command compatibility, and that a clean Ruff/pre-commit run is necessary but not enough by itself. Behaviour still needs tests and review.

After that, I think this fits the roadmap item well!