docs(readme): refresh README to reflect current state and path forward (#4)

Brings the README in line with the v2.0 Cloudflare-native architecture
shipped in the [Unreleased] CHANGELOG section and the actual feature
status documented in docs/feature-matrix.md and docs/roadmap.md.

What changed:

- Top of file now surfaces the three published SDKs (Python via PyPI,
  TypeScript via npm, LlamaIndex retriever) with install badges and
  a tabular SDK directory
- Quick-start section restructured to show SDK usage first (the
  recommended path now that they ship), with raw curl moved into the
  API reference section
- "What's in the repo" tree documents the monorepo layout — apps/
  with backend, web, sdk-ts, sdk-py, sdk-llamaindex, plus workers/
  for the edge router and Durable Objects
- Honest feature status section replaces the misleading
  "Completed" list that previously claimed Knowledge Graph, Topic
  Monitoring, Fact Verification, and Deep Research Agent were done.
  Those are 🔶 in-beta per docs/feature-matrix.md; the new section
  splits Shipped / In beta / On the roadmap to match
- Path forward section pulls the actual P0/P1/P2/P3 priorities out
  of docs/roadmap.md (MCP server is now the next P0; Python SDK is
  ✅; LangChain integration, Google OAuth, /team and /settings,
  Stripe metered overage, SSO + SOC 2 listed in order)
- API reference section bumped from "75+ endpoints" to the actual
  count (93 across 14 routers), with a headline table that flags 🔶
  endpoints inline so callers know what's beta
- Development section now covers each app in the monorepo (Python
  backend, Next.js web, edge worker, both SDKs) instead of only the
  Python backend
- Documentation index expanded with the Cloudflare architecture doc,
  honest feature matrix, ICP-ordered roadmap, runbooks, strategy
  docs, and changelog
- Self-host instructions point at the docs/quickstart.md and
  docs/deployment/ guides instead of stopping at `docker compose up`

No code touched — README-only refresh ahead of the open-source
contributor influx.

Author: Rakesh1002