docs(gsheets-logger): world-class step-by-step setup, verify & troubleshooting

[rmyndharis]

Summary

Documentation only — no code or version change. A professional rewrite of the gsheets-logger setup so a first-time user can't get stuck.

What changed

• Prerequisites + "at a glance" overview so readers see the whole arc before diving in.
• Six numbered steps with exact console breadcrumbs, direct links, expected outcomes, and GitHub [!NOTE] / [!WARNING] / [!IMPORTANT] callouts.
• The core insight, called out up front: setup grants two independent authorizations — enable the Sheets API and share the sheet with the service account — and each missing one throws a different 403 (SERVICE_DISABLED vs PERMISSION_DENIED). Fixing one only reveals the other; both are required before testing.
• New "Verify it works" section: health-endpoint buffer count to confirm events arrive, a log grep to confirm writes succeed, and the auto-retry/flush behavior.
• Troubleshooting table covering both 403s, a partial/blank JSON, a wrong spreadsheetId/sheetTab, and the rotated-key case (old key still in config → disable/re-enable).
• Treat-the-key-like-a-password warning at the point of key creation.

The generated DETAILS block is untouched (catalog:check passes); all technical sections (Features, What it logs, Configuration, Compatibility, Security) are unchanged.