docs: fix TS error in Node.js runtime local assets example (#82672)
## What?
Updated the documentation for `opengraph-image` / `twitter-image` to fix
the **Node.js runtime with local assets** example.
## Why?
The existing snippet did not type-check cleanly with Next.js 15+ and
could confuse developers.
This update improves developer experience by ensuring the snippet is
immediately usable and accurate.
## How?
- Reproduced the example in a fresh Next.js 15+ project.
- Updated the docs example to:
- Use `readFile` + Base64 encoding instead of invalid file URLs.
- Clarify that assets should be placed relative to the project root.
- Added an entry to **Version History** noting the update in v15.x.
## Checklist
- [x] Ran `pnpm prettier-fix`
- [x] Verified formatting & linting (`pnpm build && pnpm lint`)
- [x] Documentation guidelines followed
- [x] Example verified in a real Next.js 15 project
Fixes #75583 & #75625 (if considered same class of issue)
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Co-authored-by: Joseph <sephxd1234@gmail.com>
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>