docs: Improve pull request template (#2508)

## What does this PR do?

* Reorganize PR template sections for clarity.
* Add "Why?" section to explain PR purpose.
* Update instructions for linking related issues to include "Fixes".
* Add note about deleting sections if not applicable.

Author: esafak

.github/pull_request_template.md

@@ -9,25 +9,30 @@ Contribution Checklist
 
     - Fory has a strong focus on performance. If the PR you submit will have an impact on performance, please benchmark it first and provide the benchmark result here.
 -->
+## Why?
+
+<!-- Describe the purpose of this PR. -->
 
 ## What does this PR do?
 
-<!-- Describe the purpose of this PR. -->
+<!-- Describe the details of this PR. -->
 
 ## Related issues
 
 <!--
-Is there any related issue? Please attach here.
+Is there any related issue? If this PR closes them you say say fix/closes:
 
 - #xxxx0
 - #xxxx1
-- #xxxx2
+- Fixes #xxxx2
 -->
 
 ## Does this PR introduce any user-facing change?
 
 <!--
 If any user-facing interface changes, please [open an issue](https://github.com/apache/fory/issues/new/choose) describing the need to do so and update the document if necessary.
+
+Delete section if not applicable.
 -->
 
 - [ ] Does this PR introduce any public API change?
@@ -37,4 +42,6 @@ If any user-facing interface changes, please [open an issue](https://github.com/
 
 <!--
 When the PR has an impact on performance (if you don't know whether the PR will have an impact on performance, you can submit the PR first, and if it will have impact on performance, the code reviewer will explain it), be sure to attach a benchmark data here.
+
+Delete section if not applicable.
 -->