Rename Copy as prompt to Copy prompt, convert FixOption to FixCard (#95309)
## Why
Standardize the Insight fix UI across the dev overlay and docs, and
round out the docs side with a proper family index at the bottom of
every page.
## What
- Dev overlay: **Copy as prompt** -> **Copy prompt**; bumped the pill
padding to match the new docs button.
- Error pages (`errors/*.mdx`, 15 files): `<FixOption>` ->
`<FixCardGrid>`/`<FixCard>` with snippets copied verbatim from
`instant-guidance-data.ts`; all `https://preview.nextjs.org/docs/...`
URLs converted to relative `/docs/...`.
- Every Insight error page now ends with a `## Related Insights` index
listing the other 14 Insight pages (self omitted).
- `instant-navigation.mdx` and `migrating-to-cache-components.mdx`:
dropped the stray trailing `}` snippet line so guide cards match
dev-overlay heights; added a one-line Copy prompt note linking to
`#ai-workflow`.
- `insight-error-page` skill: replaced the "Useful links" guidance with
"Related Insights" (full family index, no API refs/guides) and matching
audit-checklist line.
- Minor cleanup in `instant-guidance-data.ts` (trimmed verbose JSDoc to
non-obvious bits).
## How
Paired with vercel/front#75443 which adds the Copy prompt button to
`<FixCard>` and deletes the legacy `<FixOption>`. `pnpm --filter=next
types` clean, 23 `instant-guidance-data` tests pass.
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