added the definition of done to the github templates

[joemoe]

This is a companion PR for the one i just created in the flink-web repo to introduce a definition of done: apache/flink-web#481

To increase awareness for coding standards, test and documentation we are suggesting to introduce a definition of done.

The goal of this initiative:

increase the stability of master
increase test coverage
increase awareness for code quality
improve documentation

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• 31d7883 Azure: SUCCESS

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[Airblader]

The template is starting to accumulate an awful lot of questions to answer. I would raise the question how helpful this really is, a large number of PRs don't even bother answering any of the questions and it doesn't seem to bother anyone.

At the very least we could improve the DX by making it checkboxes. Those can just be clicked and don't require moving your cursor, removing the possible answers, then adding your answer, and doing that for every single question. Something like

- [ ] I have adjusted the documentation to reflect the changes made or introduced in this pull request.

[zentol]

I don't think this is helpful.

1. is often not followed, and we don't force people to do so
2. is checked by CI (so why mention it?)
3. Why should we force documentation updates in the same PR? So long as the documentation does get updated (e.g., like in [FLINK-24876][docs] Remove metrics limitation of Adaptive Scheduler #17766) it doesn't matter.
4. is sometimes not possible/necessary
5. is checked by CI (so why mention it?)

[zentol]

Suggested change

	In order to get the change merged make sure the [Definition of Done](https://flink.apache.org/contributing/contribute-code.html#definition-of-done) is fullfilled.
	In order to get the change merged make sure the [Definition of Done](https://flink.apache.org/contributing/contribute-code.html#definition-of-done) is fulfilled.

[Airblader]

Why should we force documentation updates in the same PR?

I do think updating the docs in the same PR is a good practice and would +1 it. We merge directly to master, not to feature branches, and we should expect anything merged to master to be documented just as much as we expect it to build. I don't think that we should be treating documentation as a second-class citizen here.

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