Add path filtering to build workflow

Restrict workflow dispatch to only run on changes to:
- README.md
- parse.py
- site/ directory

This prevents unnecessary action runs when other files change.

.github/workflows/build.yml

@@ -5,6 +5,10 @@ on:
     - cron: "0 1 * * *"
   push:
     branches: [ master ]
+    paths:
+      - 'README.md'
+      - 'parse.py'
+      - 'site/**'
 
 concurrency:
   group: build-and-deploy