docs: rewrite README with current API surface and professional tone

- Reflect the 5-package workspace (was outdated: said httpx/aiohttp/requests adapters were 'planned')
- Updated examples to current API: Url.parse(), Pipeline + Policy, default_pipeline()
- Added subpackage table covering http.auth, http.sse, util, config, errors, etc.
- Added CI / Python version / mypy / ruff badges
- Tightened tone, dropped 'production-grade' hyperbole
- Trimmed Highlights to features that are actually shipped (Stage, Digest auth, retry jitter, redirect policy, etc.)
- Re-export RedirectPolicy / SetDatePolicy / their async twins from pipeline.policies so the README's API map matches real imports

Author: OmarAlJarrah