Check for dead links weekly

Due to POSIX cron syntax limitations, and GitHub not supporting `@weekly`,
a month's last run will be closer to the following month's first
than a week. Oh well, this is no big deal.

Fixes #220

.github/workflows/ci.yml

@@ -1,6 +1,10 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: '18 16 1/7 * *'
 
 jobs:
   lint-markdown: