docs: fix Pages Router fonts documentation showing App Router examples (#88326)
## Summary
Fixes #88291
The Pages Router fonts documentation was incorrectly showing App Router
examples (`app/layout.tsx`) which don't exist in Pages Router. This
confused users trying to use fonts with the Pages Router.
## Changes
Updated `docs/01-app/01-getting-started/13-fonts.mdx` to include proper
`<AppOnly>` and `<PagesOnly>` sections:
**For Pages Router users:**
- Added examples using `pages/_app.tsx` (Custom App) to wrap components
with font className
- Added examples using `pages/_document.tsx` (Custom Document) to apply
fonts to the `<html>` element
**For App Router users:**
- Wrapped existing `app/layout.tsx` examples in `<AppOnly>` sections
This follows the same pattern used in other shared documentation like
`docs/01-app/01-getting-started/11-css.mdx` which properly separates App
Router and Pages Router content.
next.js
8d7682f1 - docs: fix Pages Router fonts documentation showing App Router examples (#88326)
