docs: update contributing; add clear boundaries for issues and prs

[qcserestipy]

Closes #762

What changed

Rewrote steps 7–9 in CONTRIBUTING.md to clarify the contribution workflow and set expectations around PR quality.

Before

• Steps 7–9 were a single "Push and Open a PR" step with a link, no guidance on issue-first workflow, PR quality, or AI-generated submissions.

After

• Step 7 is now just "push to your fork" kept simple.
• Step 8 makes explicit that a dedicated issue must be opened before a PR, giving maintainers a chance to provide early feedback. Includes a bold closure policy for PRs without linked issues.
• Step 9 sets concrete PR expectations (what/why, test evidence, readable commits) and adds a note that AI-assisted work is welcome, but purely AI-generated drive-by PRs will be closed.

Why

• Contributors sometimes open PRs without issues, which makes triage harder.
• Low-effort AI-generated bulk PRs have been adding unnecessary review burden.
• The previous wording didn't set any quality bar or explain the expected workflow clearly enough.

The tone stays welcoming the goal is to encourage thoughtful contributions, not discourage new contributors.

[NucleoFusion]

lgtm
Much needed change

[Vad1mo]

Should we alo mention to research and look for existing issues and discussions prior creating a new issue and PR?