docs(styles): add sass.md docs and README docs

[joshblack]

Closes #9178

This PR is a documentation PR for:

• Updating the README for @carbon/styles
• Providing documentation on how to use tokens from the package
• Providing documentation on how to use component tokens from the package

It also includes a skeleton docs/sass.md file for describing additional entry points in the future

Changelog

New

• Add docs/sass.md for styles

Changed

• Update README for styles

Removed

Testing / Reviewing

• Read through the packages/styles/README.md file and see if it makes sense to you and is accurate
• Read through the docs available in packages/styles/docs/sass.md and see if it makes sense to you and is accurate

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[sstrubberg]

This is great. I want to use these docs to upgrade the tutorial and record my process. I'll report back anything that goes off the rails.

[andreancardona]

🎉