docs: update chat/completions documentation with real-world implementation insights (#3307)

- Add critical prerequisites: master key requirement and auth setup
- Document index chat configuration with description and documentTemplate
- Clarify PUT/PATCH methods for workspace creation
- Add authentication flow and finding Default Chat API Key
- Document enabling database search with Meilisearch tools
- Add comprehensive troubleshooting guide for common issues
- Include complete working example from setup to usage
- Fix -N flag requirement for curl streaming responses
- Update API references to match implementation

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Co-authored-by: Claude <[email protected]>

Author: tpayet