docs: add environment and item sprite generation documentation

[dev-docs-github-app-dev]

This PR enhances the getting-started documentation by adding comprehensive information about environment and item sprite generation features. The updates include:

Added Documentation for:

• Environment sprite generation with generateEnvironmentSprites()
• Item sprite generation with generateItemSprites()
• New feature listings for animation states and sprite styles
• Code examples demonstrating usage of new functions

Additional Improvements:

• Expanded feature list to include new capabilities
• Updated 'Next Steps' section with new learning paths
• Added practical code examples with configuration options

These additions will help developers better understand and utilize SpriteAI's full range of sprite generation capabilities, particularly for environment tilesets and item collections.

Resolves: team-dev-docs/spriteAI#88

[dev-docs-github-app-dev]

This pull request was created by AI Agent. Please review the changes and provide feedback. Context used:

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```

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Name	Status	Preview	Comments	Updated (UTC)
spriteai-dev-docs	❌ Failed (Inspect)			May 26, 2025 5:21pm