docs: make README public-facing for CN and BytePlus

[ark-hand]

Hand-written change from the internal SDK repository — not produced by make vendor.

• source subject: docs: make README public-facing for CN and BytePlus
• reason: Hand-written public documentation and regional examples update; not produced by ark-apis generation.

Summary

• rewrite the README for public SDK users
• document Volcengine China and BytePlus client initialization
• organize supported examples into CN and BytePlus Maven modules
• use regional Chat, Responses, embedding, image, and video model IDs
• remove retired Seededit image-edit examples
• omit the BytePlus text-only embedding example because BytePlus has no model for that endpoint
• correct BytePlus IAM comments and environment variable names
• keep MCP in both clouds and other hosted built-in tools CN-only with explicit beta headers
• add usage and manual migration guidance without internal roadmap language

Regional model defaults

• Responses and Chat: doubao-seed-2-1-pro-260628 / seed-2-0-lite-260428
• Multimodal and sparse embeddings: doubao-embedding-vision-251215 / skylark-embedding-vision-251215
• Image: doubao-seedream-5-0-pro-260628 / dola-seedream-5-0-pro-260628
• Video: doubao-seedance-2-0-fast-260128 / dreamina-seedance-2-0-fast-260128

Validation

• Maven SDK install and CN/BytePlus example module tests after rebasing onto the latest generated SDK branch
• documentation model, link, and whitespace checks

See merge request: !78

Sync-Source-Commit: c160b620adb7523914ccb0b52fc539b524e1a3a2
Hand-Written-Reason: Hand-written public documentation and regional examples update; not produced by ark-apis generation.
Release-Version: 0.4.0

Created by ark-hand.