Metadata fields improvements (#45945)
Resolves the comments from beta docs
* fix typing of `metadata.authors` rendering
* add `metadata.manifest` field
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
