docs(next): document workflow discovery and server actions

[EfeDurmaz16]

Summary

• document how the Next.js integration discovers workflows from start() calls in app/pages entries
• clarify that Server Actions can call start(), while workflow-imported modules should not carry top-level "use server"
• add a small split-wrapper example for Server Actions and workflow logic

Closes #817.

Verification

• pnpm install
• pnpm --filter docs lint:links
• pnpm --filter docs build (blocked by existing prerender failure on /en/v5/docs/api-reference/workflow-api/get-hook-by-token: generateDefinition.code resolved as any; build reached this unrelated docs page after compiling successfully)

Notes

The local environment warns that Node v26.0.0 is outside the repo supported engine range (^18 || ^20 || ^22 || ^24).

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