ci(pull_request_template.md): added PR template

.github/pull_request_template.md

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+## ✅ What
+ 
+<!-- A brief description of the changes in this PR. -->
+ 
+## 🤔 Why
+ 
+<!-- A brief description of the reason for these changes. -->
+
+## 🧪 How was this tested
+
+<!-- How were your changes incorporated into the testing framework -->
+ 
+## 👩‍🔬 How to validate
+ 
+<!-- Step-by-step instructions for how reviewers can verify these changes work as expected. -->
+
+## 🥇 Is a review required
+
+<!-- Is this a change that requires external review or can you do the PR yourself -->
+ 
+## 🔖 Further reading
+
+<!-- Does this change reference any papers or external readings that are important? -->