security: pin dorny/paths-filter action to commit SHA

- Replace dorny/paths-filter@v3 with full commit SHA (de90cc6fb38fc0963ad72b210f1f284cd68cea36)
- Pinning to commit SHA prevents supply chain attacks via tag/branch manipulation
- Added comment indicating this is v3 for maintainability
- Follows GitHub security best practices for action dependencies

Author: RossBugginsNHS

.github/workflows/docs-cicd-1-pull-request.yaml

@@ -24,7 +24,7 @@ jobs:
     steps:
     - name: "Checkout code"
       uses: actions/checkout@v5
-    - uses: dorny/paths-filter@v3
+    - uses: dorny/paths-filter@de90cc6fb38fc0963ad72b210f1f284cd68cea36 # v3
       id: changes
       with:
         filters: |