docs(cli): document backend framework selection

[AbhinRustagi]

What

Documents --backend-framework across the CLI reference and the package and template READMEs.

Changes

• Document the flag and its values, and that the framework prompt follows the template prompt — non-interactive runs default to default.
• Add a matrix mapping each value to the route it produces for the Cloud and self-hosted templates.
• Add a Conversation storage section: Cloud is the durable store for every Cloud variant, while self-hosted keeps messages in memory for the page session only.
• Deployment guidance points at Vercel throughout.
• Fix the Cloud template README, which told people to cp .env.example .env.local — the CLI writes .env, and no .env.example ships.

Docs only. Migrated from #785.

Test Plan

• Verified locally

prettier --check clean, which is what docs CI gates on cli.mdx. Cross-checked every behavioural claim against the code in #964 — the default fallback, the prompt ordering, and the THESYS_API_KEY / OPENAI_API_KEY gate.

Checklist

• I updated docs/README when needed
• I considered backwards compatibility

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Project	Deployment	Actions	Updated (UTC)
openui-docs	Ready	Preview	Aug 24, 2026 9:00am