[SPARK-28578][INFRA] Improve Github pull request template

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### What changes were proposed in this pull request?
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This PR proposes to improve the Github template for better and faster review iterations and better interactions between PR authors and reviewers.

As suggested in the the [dev mailing list](http://apache-spark-developers-list.1001551.n3.nabble.com/DISCUSS-New-sections-in-Github-Pull-Request-description-template-td27527.html), this PR referred [Kubernates' PR template](https://raw.githubusercontent.com/kubernetes/kubernetes/master/.github/PULL_REQUEST_TEMPLATE.md).

Therefore, those fields are newly added:

```
### Why are the changes needed?
### Does this PR introduce any user-facing change?
```

and some comments were added.

### Why are the changes needed?
<!--
Please clarify why the changes are needed. For instance,
  1. If you propose a new API, clarify the use case for a new API.
  2. If you fix a bug, you can clarify why it is a bug.
-->

Currently, many PR descriptions are poorly formatted, which causes some overheads between PR authors and reviewers.

There are multiple problems by those poorly formatted PR descriptions:

- Some PRs still write single line in PR description with 500+- code changes in a critical path.
- Some PRs do not describe behaviour changes and reviewers need to find and document.
- Some PRs are hard to review without outlines but they are not mentioned sometimes.
- Spark is being old and sometimes we need to track the history deep. Due to poorly formatted PR description,  sometimes it requires to read whole codes of whole commit histories to find the root cause of a bug.
- Reviews take a while but the number of PR still grows.

This PR targets to alleviate the problems and situation.

### Does this PR introduce any user-facing change?
<!--
If yes, please clarify the previous behavior and the change this PR proposes - provide the console output, description and/or an example to show the behavior difference if possible.
If no, write 'No'.
-->

Yes, it changes the PR templates when PRs are open. This PR uses the template this PR proposes.

### How was this patch tested?
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If tests were added, say they were added here. Please make sure to add some test cases that check the changes thoroughly including negative and positive cases if possible.
If it was tested in a way different from regular unit tests, please clarify how you tested step by step, ideally copy and paste-able, so that other reviewers can test and check, and descendants can verify in the future.
If tests were not added, please describe why they were not added and/or why it was difficult to add.
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Manually tested via Github preview feature.

Closes apache#25310 from HyukjinKwon/SPARK-28578.

Lead-authored-by: HyukjinKwon <[email protected]>
Co-authored-by: Hyukjin Kwon <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: HyukjinKwon

.github/PULL_REQUEST_TEMPLATE

@@ -1,10 +1,42 @@
-## What changes were proposed in this pull request?
+<!--
+Thanks for sending a pull request!  Here are some tips for you:
+  1. If this is your first time, please read our contributor guidelines: https://spark.apache.org/contributing.html
+  2. Ensure you have added or run the appropriate tests for your PR: https://spark.apache.org/developer-tools.html
+  3. If the PR is unfinished, add '[WIP]' in your PR title, e.g., '[WIP][SPARK-XXXX] Your PR title ...'.
+  4. Be sure to keep the PR description updated to reflect all changes.
+  5. Please write your PR title to summarize what this PR proposes.
+  6. If possible, provide a concise example to reproduce the issue for a faster review.
+-->
 
-(Please fill in changes proposed in this fix)
+### What changes were proposed in this pull request?
+<!--
+Please clarify what changes you are proposing. The purpose of this section is to outline the changes and how this PR fixes the issue. 
+If possible, please consider writing useful notes for better and faster reviews in your PR. See the examples below.
+  1. If you refactor some codes with changing classes, showing the class hierarchy will help reviewers.
+  2. If you fix some SQL features, you can provide some references of other DBMSes.
+  3. If there is design documentation, please add the link.
+  4. If there is a discussion in the mailing list, please add the link.
+-->
 
-## How was this patch tested?
 
-(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
-(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)
+### Why are the changes needed?
+<!--
+Please clarify why the changes are needed. For instance,
+  1. If you propose a new API, clarify the use case for a new API.
+  2. If you fix a bug, you can clarify why it is a bug.
+-->
 
-Please review https://spark.apache.org/contributing.html before opening a pull request.
+
+### Does this PR introduce any user-facing change?
+<!--
+If yes, please clarify the previous behavior and the change this PR proposes - provide the console output, description and/or an example to show the behavior difference if possible.
+If no, write 'No'.
+-->
+
+
+### How was this patch tested?
+<!--
+If tests were added, say they were added here. Please make sure to add some test cases that check the changes thoroughly including negative and positive cases if possible.
+If it was tested in a way different from regular unit tests, please clarify how you tested step by step, ideally copy and paste-able, so that other reviewers can test and check, and descendants can verify in the future.
+If tests were not added, please describe why they were not added and/or why it was difficult to add.
+-->