docs: organize Docker setup into a clear reading order

[dadachann]

Summary

• Organize each Docker guide as prepare, download, start, and open the dashboard.
• Use editable multiline commands and only one service startup command.
• Put mainland-China image replacement instructions before startup.
• Fold optional public-server token configuration before startup, persist values in .env, and remove internal runtime details and the basic-only detour.
• Remove SeekDB details from this beginner page; retain links to follow-up model, bot, sandbox, and skill guides.
• Add locale-aware tests for ordering, multiline commands, mirror placement, and removal of unrelated configuration.

Validation

Node 24: npm test, npm run typecheck, npm run build including static-output tests; git diff --check.

Related to langbot-app/LangBot#2463.