docs: sync node credentials and dashboard webhooks

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Summary

• document the gt-node credential environment fallback prepared for gt-node@1.0.8
• document webhook Events delivery trigger, latency, transport errors, and retry navigation
• correct free-plan/signup-credit wording and static data-export eligibility
• restore the missing translation-review guide introduction and clarify plain-text formatting

Findings fixed

• Broken: Node docs denied the new credential fallback; webhook retry/history pointed to the wrong Dashboard page; one recently edited guide had no introduction.
• Wrong: Webhook navigation and button labels, billing credit semantics, and static-export eligibility were inaccurate or incomplete.
• Polish: Reference overview tables, workflow terminology, and plain-text behavior wording now follow DOCS-SKILL.md.

Validation

• pnpm build:content
• content validator unit tests
• link validation across 372 content files
• unsafe HTML validation across 418 MDX files
• focused gt-node runtime credential tests (6 passed)
• fresh npm project with gt-node@latest

Release dependency

As of the audit, npm still resolves gt-node@latest to 1.0.7; the environment fallback is in merged source and the open gt-node@1.0.8 release PR (generaltranslation/gt#1931). Land this content change with or after that release.