Move Accordion to Core

[alycrys]

A component is complete and ready for consumption (moved to core) when all the following are checked:

• Stories for all features/variations
• Add QA story
• Add Custom styling story
• Jest tests migrated to Cypress
• Tests added for all features/variations
• Check CSS variables (characteristic/foundation/token etc) are used correctly
• ADA review done

Docs added:

• Props table
• HTMxL anatomy
• CSS class
• Characteristics used
• Custom property API
• Customising styles

Migrating tips:

• Mention breaking changes
• Any useful advice or mention

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Latest commit: 8eb4e46

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[joshwooding]

Closing this for now since Accordion isn't high priority.