Add resource to learn zk via game in index.md

[hackertron]

• add zk game learning resource : zk-cards

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[github-actions]

This issue is stale because it has been open 30 days with no activity.

[wackerow]

Lgtm, thanks @hackertron!

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[wackerow]

@all-contributors please add @hackertron for tool

[allcontributors]

@wackerow

I've put up a pull request to add @hackertron! 🎉