Change document format to markdown from xdoc

[kazuki43zoo]

I will consider to change document format because maven-site-plugin supports markdown format. The markdown format can render on GitHub.

• https://github.com/mybatis/spring-boot-starter/blob/master/mybatis-spring-boot-autoconfigure/src/site/markdown/index.md

• https://github.com/mybatis/spring-boot-starter/blob/master/mybatis-spring-boot-test-autoconfigure/src/site/markdown/index.md