docs(glossary): Add definitions for "Design System" and "Design Token" to Glossary

[litteredwitherrors]

What feature are you requesting?

Add clear definitions of what a "Design System" and "Design Token" to the FAST website's documentation glossary.

What would be the benefit of adding this feature?

These entries would help to bring clarity to each term to reduce confusion. Entries can be referenced throughout the rest of the documentation site.

Is there any additional context for your request?
#3738 (comment)

[nicholasrice]

I've added Design System to #3747. I didn't add Design Token because we don't actually use that term anywhere in code; we refer to Design Tokens as "design system properties".

I do think it could be good to align terminology toward the industry terms. We use #3361 as an opportunity to align to "Design Tokens" in code