gh: collapse checks in PR template (#56)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,10 +1,13 @@
-# Before submitting
+<details>
+  <summary><b>Before submitting</b></summary>
 
 - [ ] Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
 - [ ] Did you read the [contributor guideline](https://github.com/Lightning-AI/pytorch-lightning/blob/main/.github/CONTRIBUTING.md), Pull Request section?
 - [ ] Did you make sure to update the docs?
 - [ ] Did you write any new necessary tests?
 
+</details>
+
 ## What does this PR do?
 
 Fixes # (issue).