update github action to run every quarter

Author: mr8lu

.github/workflows/check-links-self.yaml

@@ -3,6 +3,8 @@ on:
   workflow_dispatch:
   push:
   pull_request:
+  schedule:
+    - cron: '0 0 1 */3 *'
 
 jobs:
   link_check:

.github/workflows/notebook-lint-self.yaml

@@ -2,6 +2,8 @@ name: 'Lint Notebook'
 on:
   push:
   workflow_dispatch:
+  schedule:
+    - cron: '0 0 1 */3 *'
 permissions:
   contents: write
   id-token: write