docs: publish SDK and beacon to npm, add onboarding guide

- @inventhq/tracker-sdk@0.1.0 published to npm (server-side SDK)
- @inventhq/tracker-beacon@0.1.0 published to npm (browser beacon)
- Updated SDK README with @inventhq scope and full API docs
- Created detailed beacon README with privacy design, attribution flow
- Created docs/onboarding.md — full integration guide for developers and AI agents

Author: V V