[Feature][Document] Add example and notice about task type MapReduce

[zhongjiajie]

Due to our tasks task description is not containing some detail, and I think we should add a new section notice and give an example to the task map-reduce in the document https://dolphinscheduler.apache.org/en-us/docs/latest/user_doc/guide/task/map-reduce.html

[zhongjiajie]

I add a PR in apache/dolphinscheduler-website#638, maybe someone who is interested in this feature could take it as an example

[QuakeWang]

Please assign it to me, I will do it later.

[zhongjiajie]

@QuakeWang Looking forward your contribution

[QuakeWang]

This task has been completed.

ref: apache/dolphinscheduler-website#669

[zhongjiajie]

closed by apache/dolphinscheduler-website#669
FYI @QuakeWang, I add some suggestions about how to relate PR and issue in apache/dolphinscheduler-website#669 (comment)