Add trigger

The workflow for deploying the documentation wasn't triggered on PRs.
In that case, the PR's status check will keep waiting infinitely.

.github/workflows/deploy-docs.yml

@@ -4,6 +4,9 @@ on:
   push:
     branches:
       - main
+  pull_request:
+    branches:
+      - main
 
 permissions:
   contents: write