Docs - Add `screen-breakpoints` in container

[biewdev]

...
<div class="container is-(breakpoint)">
...

$screen-breakpoints: (
	xxl: 1400px,
	xl: 1280px,
	lg: 1024px,
	md: 768px,
	sm: 640px,
	xs: 480px,
	xxs: 360px,
);