[Doc][Workflow] Add docs for workflow execution type

[EricGao888]

Search before asking

• [X] I had searched in the issues and found no similar feature requirement.

Description

• Currently we have no docs to explain what these four workflow execution types mean. It is necessary that we have some brief explanations for them in docs.

https://github.com/apache/dolphinscheduler/blob/7c0c1f4b78ffafec7f4afa8758d41b8fc8c9db8f/dolphinscheduler-common/src/main/java/org/apache/dolphinscheduler/common/enums/ProcessExecutionTypeEnum.java#L24-L30

Documentation Links

• https://dolphinscheduler.apache.org/en-us/docs/latest/user_doc/guide/project/workflow-definition.html

Are you willing to submit a PR?

• [x] Yes I am willing to submit a PR!

Code of Conduct

• [X] I agree to follow this project's Code of Conduct

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[EricGao888]

WIP