docs: add DomainKit SDK and React guides (#36)

## Summary

Turns the Blume shell into a reviewable DomainKit reference: quickstart,
provisioning lifecycle, provider connections, verification, Cloudflare,
Vercel, and React integration.

## Navigation

```text
Introduction
├── Quickstart
├── Core concepts
│   ├── Plans and receipts
│   ├── Provider connections
│   └── Verification
├── Providers
│   ├── Cloudflare
│   └── Vercel
└── React
    ├── Overview
    ├── Host transport
    └── Composition and theming
```

## Screenshots

![DomainKit documentation navigation and
introduction](https://scratchpad.arya.sh/f/01a05287-41fd-77e7-b515-607a2d608501/domainkit-docs-content.png)

## Validation

- `bun run docs:build`
- `bun run --filter @domainkit/docs typecheck`
- `bun run docs:audit` (912 audits, 0 findings)
- `bun run lint`
- `bun run format:check`
- Browser review of the docs navigation and rendered content
- Local Orama/WebMCP search for `Cloudflare OAuth`