[ChatStateLayer] Overview markdown document

[laevandus]

🔗 Issue Links

Related: #728

Merges to feature/chat-state-layer

🎯 Goal

A concise overview of the new state layer which introduces new types but not going into great detail.

☑️ Contributor Checklist

• [x] I have signed the Stream CLA (required)
• [x] This change follows zero ⚠️ policy (required)
• [ ] This change should be manually QAed
• [ ] Changelog is updated with client-facing changes
• [ ] New code is covered by unit tests
• [ ] Comparison screenshots added for visual changes
• [ ] Affected documentation updated (docusaurus, tutorial, CMS)

🎁 Meme

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[laevandus]

Overall looks great 👌 Just added some comments to discuss the new API. The documentation is really helpful to re-review the new API 🙂

Oh, just merged it without noticing. Will check your comments tomorrow.