chore: update contribution guide && modify mybatis-plus module structure #51

Job	Run time
basic (go-sql-driver, golang, true) / ci	1m 31s
basic (mybatis-plus, java, false) / ci	1m 40s
basic (mysql-connector-python, python, true) / ci	1m 24s
basic (mysql-connector-java, java, true) / ci	1m 36s
basic (oceanbase-client, java, true) / ci	1m 38s
basic (pymysql, python, true) / ci	1m 19s
basic (testcontainers-java, java, false) / ci	1m 37s
	10m 45s

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