docs(gatsby-guide): update usage and include starter

[TylerAPfledderer]

Closes #1549

📝 Description

Updates the Gatsby Guide Page to

• Include installation with Gatsby starter
• Update usage example to the latest implementation

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Name	Status	Preview	Updated (UTC)
chakra-ui-docs	✅ Ready (Inspect)	Visit Preview	Jun 6, 2023 5:29pm