[BD-46] feat: add initial documentation

[monteri]

Description

Add documentation for Design Tokens in README.md and in the Design Tokens page in the Foundation section

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (feat/design-tokens@1ad0236). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head 8afdf1d differs from pull request most recent head d52e388. Consider uploading reports for the commit d52e388 to get more accurate results

Additional details and impacted files

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[openedx-webhooks]

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