Write initial usage docs for @carbon/styles

[joshblack]

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

• How to bring in the package and use the styles
• How to bring in individual component styles
• How to change the theme
• How to use design tokens (overlap with Design Tokens)
• How to bring in IBM Plex to your project

These docs should be present in the README.md and docs/sass.md files in the packages/styles folder. The README.md should contain high-level, common information and the docs/sass.md file will be comprehensive/contain all information.

In other words, README.md should contain high-level usage/examples. docs/sass.md should contain detailed API information