Stubtest docs: common failure/suppression examples + debugging guidance

I feel like the contributing guide could benefit from instructions around debugging stubtest failures, and examples of things that stubtest can't handle that would warrant suppressions.

The mypy stubtest docs don't really go into enough detail or have enough examples, which makes it harder for new contributors (like myself) to debug failures.

Maybe we could improve that doc and link to it from CONTRIBUTING.md. I think that would reduce the need for maintainers to help stub contributors debug these failures, and make things a bit more self-service.

I'm happy to contribute to this, though I'm a bit too new to know which specific patterns are commonly suppressed.

CONTRIBUTING.md とリンク先の mypy stubtest ドキュメントから始め、stubtest のデバッグと抑制に関するガイダンスをどこに置くべきかを特定します。一般的な失敗例と抑制例を追加し、失敗をデバッグする方法を説明し、改善したドキュメントへのリンクを CONTRIBUTING.md から追加すれば完了です。