fix(docs): imperative and target links in homepage

[heyfirst]

Why

It seems the link buttons in document is broken. Both imperatives and target. Also, it seems target concept document is not introduce yet but I believe it would be under /docs/concept/target for sure.

What

Correct both of incorrect links.

Checklist

• Documentation updated
• Demo added
• Ready to be merged

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Latest commit: 1839048

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Name	Status	Preview	Updated
react-spring-io	❌ Failed (Inspect)		Oct 15, 2022 at 9:22AM (UTC)

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Sandbox	Source
spring-animating-auto	Configuration
spring-card	Configuration
spring-cards-stack	Configuration
spring-chain	Configuration
spring-css-keyframes	Configuration
spring-draggable-list	Configuration
spring-exit-before-enter	Configuration
spring-flip-card	Configuration
spring-goo-blobs	Configuration
spring-image-fade	Configuration
spring-list-reordering	Configuration
spring-masonry	Configuration
spring-multistage-transition	Configuration
spring-notification-hub	Configuration
spring-notification-hub	Configuration
spring-notification-hub	Configuration
initial-rocket	Configuration
spring-simple-transition	Configuration
spring-svg-filter	Configuration
spring-slide	Configuration
spring-tree	Configuration
spring-viewpager	Configuration