Update API page UI

[ACrazyTown]

The API page is still using parts of the old HaxeFlixel website (observe the header and footer). It would be nice to update this to match every other page.

I took a quick peek myself and it doesn't seem to be very straightforward. The HaxeFlixel website uses Bootstrap v5, and while Haxe's Dox tool bundles Bootstrap with the pages it generates, it's an incredibly old version (v1 from 2013). Seems like this was "fixed" before by just copying newer Bootstrap CSS into the extra-styles.css file. Maybe this could be done again, or there could be work done on the Dox side to update it to use newer versions of Bootstrap.