optimize: add docker-compose for AT samples and improve README

[WangzJi]

Ⅰ. Describe what this PR did

1. Adding [docker-compose.yaml] to at-sample/ directory
   • Starts MySQL (with tables initialized), Zookeeper, and Seata Server
   • One command to set up all infrastructure for any AT sub-sample
2. Adding shared SQL init script (at-sample/sql/init.sql)
   • Creates account_tbl, stock_tbl, order_tbl, and undo_log tables
   • Includes test data for the springboot-dubbo-seata sample

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[slievrly]

LGTM