Add explanation to deploy workflow

Explain why we need to checkout deploy files from master

Author: flip1995

.github/workflows/deploy.yml

@@ -39,7 +39,10 @@ jobs:
       if: github.ref == 'refs/heads/beta'
       run: echo "BETA=true" >> $GITHUB_ENV
 
-    - name: Use deploy script from master branch
+    # We need to check out all files that (transitively) depend on the
+    # structure of the gh-pages branch, so that we're able to change that
+    # structure without breaking the deployment.
+    - name: Use deploy files from master branch
       run: |
         git fetch --no-tags --prune --depth=1 origin master
         git checkout origin/master -- .github/deploy.sh util/versions.py util/gh-pages/versions.html