docs: remove incorrect _document.tsx font example

[zamadye]

saw the issue about the _document.tsx example not working. next/font cant be used in _document.tsx, it throws an error. removed that section from the docs since _app.tsx already has the correct example.

fixes #88832

[nextjs-bot]

Allow CI Workflow Run

• approve CI run for commit: b4a759c

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[nextjs-bot]

Allow CI Workflow Run

• approve CI run for commit: b4a759c

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[icyJoseph]

Ty for the PR:

• next.js/packages/next/src/build/webpack/loaders/next-font-loader/index.ts

  Lines 32 to 36 in 68dade4

  	// Throw error if @next/font is used in _document.js
  	if (/pages[\\/]_document\./.test(relativeFilePathFromRoot)) {
  	const err = new Error(
  	`${bold('Cannot')} be used within ${cyan('pages/_document.js')}.`
  	)

[zamadye]

tried but tests are failing. any suggestions?