Write usage docs for CSS Grid from @carbon/styles

[tay1orjones]

This issue is for writing out the initial usage docs for the @carbon/styles package. These usage docs should include:

• What import to use
• Basic API overview, mixins and class names
• Only include a minimal functional (basic grid) example, no grid mode switching or hanging, point to the full grid usage docs (packages/grid/readme.md)

This info could be mentioned in packages/styles/readme.md, but really should probably live in the more comprehensive packages/styles/docs/sass.md.