docs: storage-setup guide omits the registerStorageProvider step — following it throws 'Unknown storage provider type' at upload

[borisno2]

Summary

docs/content/guides/storage-setup.md (at the 0.24.0 tag) shows configuring providers via s3Storage(...) / vercelBlobStorage(...) in the config, but never mentions registerStorageProvider from @opensaas/stack-storage/runtime (added in #551). Since only 'local' is built into the registry (runtime/registry.js:76-80), a host that follows the guide verbatim gets a working READ path and then throws at the first upload:

Unknown storage provider type: s3. Register it with registerStorageProvider('s3', ...)

The registration step is currently documented only in the package CLAUDE.md and the CHANGELOG.

Ask

Add the registration step to the storage-setup guide for each non-local provider, e.g.:

import { registerStorageProvider } from '@opensaas/stack-storage/runtime'
import { vercelBlobStorage, VercelBlobStorageProvider } from '@opensaas/stack-storage-vercel'

registerStorageProvider('vercel-blob', (c) => new VercelBlobStorageProvider(c))

…and note WHERE to call it (a server-only module evaluated before any upload — worth an explicit recommendation for Next.js hosts).

Context

Hit while wiring @opensaas/stack-storage-vercel@0.24.0 for image upload in a Keystone→stack migration (the SF-15 / #549 follow-through).