docs: remove incorrect _document.tsx font example (#91346)
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
Co-authored-by: Kahfi Elhady <kahfie@gmail.com>
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
