Render PR checklist in details to hide it (#1037)

Author: spacebear21

.github/pull_request_template.md

@@ -1,9 +1,10 @@
-## Pull Request Checklist
+<details>
+  <summary>Pull Request Checklist</summary>
 
 Please confirm the following before requesting review:
 
 - [ ] I have [disclosed my use of
   AI](https://github.com/payjoin/rust-payjoin/blob/master/.github/CONTRIBUTING.md#ai-assistance-notice)
   in the body of this PR.
 - [ ] I have read [CONTRIBUTING.md](https://github.com/payjoin/rust-payjoin/blob/master/.github/CONTRIBUTING.md#commits) and **rebased my branch to produce [hygienic commits](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md#committing-patches)**.
-
+</details>