Rebuild when the binder requirements file changes

.github/workflows/sphinx.yml

@@ -4,7 +4,7 @@ on:
   workflow_dispatch:
   push:
     branches: ["main"]
-    paths: ["docs/**", "README.md", "CHANGELOG.md"]
+    paths: ["docs/**", "README.md", "CHANGELOG.md", "requirements.txt"]
 
 permissions:
   contents: "read"