new PR template (#2191)

## Summary
Why:  Improve PR quality and review-ability.
What:  modifies current PR template to be structured and require more details when submitting PRs.

## Expected Behavior
PR must come with sufficient details to address or explain the issue.

## Actual Behavior
PR template only requires link to the issue:

```
This PR resolves #<Replace-Me-With-The-Issue-Number-Addressed-By-This-PR>.
```

## Steps to reproduce
1. either create a new PR or
2. see [the current template](https://github.com/linkedin/cruise-control/blob/c5545ef04618b5b42290edda2ee63eb6bfa2e1a6/docs/pull_request_template.md)

## Known Workarounds
People voluntarily provide additional details

## Additional Evidence
- n/a

## Categorization
- [x] refactor

docs/pull_request_template.md

@@ -1 +1,31 @@
-This PR resolves #<Replace-Me-With-The-Issue-Number-Addressed-By-This-PR>.
+## Summary
+1. Why: <!-- describe the motivation -->.
+2. What: <!-- describe introduced changes -->.
+
+## Expected Behavior
+…
+
+## Actual Behavior
+…
+
+## Steps to Reproduce
+1. …
+2. …
+
+## Known Workarounds
+<!-- if any -->
+
+## Additional evidence
+1. <!-- your environment -->
+2. <!-- logs --> 
+3. <!-- etc… -->
+4. <!-- use <detail> tag for large snippets -->
+
+## Categorization
+- [ ] bugfix
+- [ ] new feature
+- [ ] refactor
+- [ ] CVE
+- [ ] other
+
+This PR resolves #<Replace-Me-With-The-Issue-Number-Addressed-By-This-PR> if any.