Add a section on changing fonts to PDF docs

[cwickham]

E.g. give example of using pdf font options

Right now, might be best after (or as part of): https://quarto.org/docs/output-formats/pdf-basics.html#output-options

Suggestion from #4390 :

I think it would be very helpful to give a specific complete example (very brief) where the fonts look like the default LaTeX fonts.

There is an example buried in https://quarto.org/docs/output-formats/pdf-basics.html#unicode-characters

Make sure to call out limitation to xelatex and lualatex only.