Remove page-specific font example from docs (#34694)
The Font Optimization docs have an outdated example that recommends
adding fonts via `next/head`. This is an anti-pattern that Next.js
has already warned about through ESLint for a while (see
https://nextjs.org/docs/messages/no-page-custom-font) and is now
starting to warn about through the console because it won't work
well with streaming architecture.
This change removes the outdated example from the docs.
Fixes #34693
## Documentation / Examples
- [x] Make sure the linting passes by running `yarn lint`
