[BB-9986] docs: Revamp design tokens docs

Kshitij Sobtirequested to merge
kshitij/design-tokens into master

This PR revamps the documentation about design tokens, adding a lot more detail and making it easier for people to get up to date and started with theming MFEs.

Ref: https://tasks.opencraft.com/browse/BB-9986

Edited by Kshitij Sobti

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