Trigger docs workflow on pull-requests

Author: matt-graham

.github/workflows/docs.yml

@@ -1,10 +1,23 @@
 name: Docs
 
 on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - .github/workflows/docs.yml
+      - pyproject.toml
+      - s2fft/**
+      - docs/**
+      - notebooks/**
   push:
     branches:
     - main
-    
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
+
 jobs:
   build: