github: Run workflows weekly

In case upstream, dependency, network or other issues cause failures

Signed-off-by: Philip Molloy <philip@philipmolloy.com>

Author: pamolloy

.github/workflows/gh-pages.yml

@@ -5,6 +5,8 @@ on:
     branches:
       - main
   pull_request:
+  schedule:
+    - cron: '0 0 * * 0'
 
 jobs:
   deploy:

.github/workflows/markdownlint.yml

@@ -5,6 +5,8 @@ on:
       - main
   pull_request:
   workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * 0'
 
 jobs:
   build:

.github/workflows/vale.yml

@@ -5,6 +5,8 @@ on:
       - main
   pull_request:
   workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * 0'
 
 jobs:
   build: