Add Font Awesome to Icons page (Extend section)

Created by: coliff

On the Migration page (https://getbootstrap.com/docs/4.0/migration/) it's suggested to use Font Awesome as a potential replacement to the Glyphicons (which were included in Bootstrap 3), however on the Icons page in Extend section (https://getbootstrap.com/docs/4.0/extend/icons/) Font Awesome isn't mentioned.

Would you accept a PR with them added? They are very popular and work well. If we added them to the page would they be 'Preferred' or 'More Options' ?