docs: Next.js 15 RC updates (#67839)
[DOC-3018](https://linear.app/vercel/issue/DOC-3018/nextjs-15-rc-updates)
New APIs:
- Fetch HMR cache
- Static Indicator
- `onRequestError`
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Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>
Co-authored-by: JJ Kasper <jj@jjsweb.site>
