Remove Fira font files

* Remove `font/` directory with Fira fonts
* Remove corresponding CSS references to Fira fonts

https://trello.com/c/abHQcg6W
Resolve Safari SVG rendering

* Remove old Middleman brand assets
* Move `middleman-logo.svg` outside of `logos/`
* Modify box-shadow value of global nav style

https://trello.com/c/YdBlvKwh
Improve visual design of documentation navigation

* Remove `border` from documentation navigation
* Add `margin` below `global-nav` so it's modular and in `content-container`
* Add margin to hero content in lieu of margin from global nav
* Refactor with container names

https://trello.com/c/KNHdE47Q