feat: Add a nightly build

Added a scheduled nightly build workflow to keep the deployed site current
with any content changes, particularly in the private submodule.

.forgejo/workflows/deploy.yaml

@@ -5,6 +5,8 @@ on:
     branches:
       - master
       - develop
+  schedule:
+    - cron: '0 2 * * *'
 
 jobs:
   deploy: