docs: add Docker Compose quick start to README

[me0106]

Summary

• Move Docker Compose to the front of the README as the default quick start path.
• Add Docker startup instructions for memind-server and memind-ui, including ports, health check, logs, shutdown,
  and persisted data volume.
• Clarify that OPENAI_API_KEY is required, while OPENAI_BASE_URL, OPENAI_CHAT_MODEL, and
  OPENAI_EMBEDDING_MODEL are optional, and that chat/embedding model choices affect Memory quality.
• Keep the original Java quickstart and embedding instructions as the source-based path.

Test Plan

• Ran git diff --check -- README.md README_zh.md
• Verified README and README_zh content structure manually

Notes

This is a documentation-only change. No runtime code was changed.