docs: add Payload CMS npm setup guide

[tin-computer]

Summary

• add a focused Payload CMS npm setup and repair guide
• cover new app creation, existing Next.js setup, exact version alignment, and duplicate dependency fixes
• surface the guide in docs navigation and link to it from the installation guide
• protect the page's search intent, commands, links, and sitemap discovery with a focused regression test

Search evidence

The fixed July 5–August 3 Search Console cohort disclosed four impressions across payload npm, npm payloadcms, and payloadcms npm, all with zero clicks. payload configuration remains reserved as the next distinct topic with one impression and zero clicks.

Scope

This page stops at getting Payload itself installed and healthy. The existing installation guide continues to own Payload Components CLI setup, while the architecture, registry, seed, forms, admin-components, and custom-components boundaries remain unchanged.

Verification

• pnpm exec vitest run --config ./vitest.config.mts tests/int/fumadocs-site.int.spec.ts (33/33)
• pnpm format:check
• pnpm source:build
• pnpm exec tsc --noEmit
• pnpm lint
• pnpm build
• local route returned HTTP 200 and passed a visual review

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Project	Deployment	Actions	Updated (UTC)
payload-components	Ready	Preview	Aug 17, 2026 10:11pm
payload-components-97bf	Ready	Preview	Aug 17, 2026 10:11pm