[docs] Improve first dry-run troubleshooting

[stdiodh]

Goal

Improve the first dry-run troubleshooting path for fork users.

Context

The fork setup guide already explains dry-run and artifact review. A small contributor can make it easier to diagnose the first failed run without changing workflow behavior.

Suggested work

• Review docs/kr/getting-started/fork-setup.md and docs/en/getting-started/fork-setup.md.
• Add or clarify one troubleshooting entry for first dry-run failures.
• Keep examples secret-safe and avoid real webhook URLs, tokens, account names, or private Discord details.

Acceptance criteria

• The troubleshooting entry is short and actionable.
• Korean and English docs stay aligned.
• python3 scripts/check-doc-format.py passes.
• No generated reports/ files are committed.