Separate What and Why in the PR template

[t-kikuc]

What this PR does / why we need it:

• as title
• What for?
  • Easier to distinguish which sentences are WHAT / WHY
  • Force committers to write WHY

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 24.31%. Comparing base (4824dc2) to head (5ae61b6). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5278 +/- ## ========================================== + Coverage 24.28% 24.31% +0.03% ========================================== Files 440 440 Lines 46754 46776 +22 ========================================== + Hits 11352 11374 +22 Misses 34499 34499 Partials 903 903 ```

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