[FLINK-29424] Add Pull Request Template

[jiangxin369]

What is the purpose of the change

Add Pull Request Template for Flink ML

Brief change log

• Add Pull Request Template for Flink ML

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[yunfengzhou-hub]

Thanks for the PR. Could you please refractor this PR's description? Changing

If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

to

If yes, how is the feature documented? (not applicable)

The rest LGTM.

[lindong28]

Thanks for the PR! LGTM.