Improve SDK README structure and product status

[Borets]

Summary

• reorganize the root README around SDK selection, supported products, installation, examples, and support
• remove the SDK-wide Early Access warning and scope Early Access status to Sandboxes and Object Storage
• add clearer Python and TypeScript requirements, authentication guidance, and product documentation links
• remove stale repository-layout and development details from the root README

Validation

• git diff --check
• verified that every relative file link in the README resolves

Documentation-only change; no SDK code or package behavior is modified.