docs(thermidor): Add why.md

[lavoiesl]

While trying to understand thermidor, I wanted to better understand the why and got Kiro to assemble this such that we can align on it.

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commit: c5aa298

[Copilot]

Pull request overview

Adds a new Thermidor documentation article explaining the motivation for creating @coveo/thermidor, contrasting it with today’s @coveo/headless architecture, and framing the shift toward unified multi-API experiences and server-progressive (streaming) generative interfaces.

Changes:

• Introduces a “Why Thermidor?” doc describing Headless’s current multi-engine constraints and the resulting complexity for hybrid experiences.
• Explains the architectural shift from client-driven request/response to server-progressive streaming, including Mermaid diagrams.
• Summarizes Thermidor’s design responses (single store, slice adoption, interfaces/facades, hydration) and links to related Thermidor docs.

[jfcere-coveo]

Thermidor - One store, sliced by feature, scoped by interface

Suggested change

	| **State stores** | One Redux store per use case (Search, Commerce, etc.) | One store, multiple interfaces |
	| **State stores** | One Redux store per use case (Search, Commerce, etc.) | One store, sliced by feature, scoped by interface |