Add scheduled regeneration CI jobs

This mirrors what Homebrew/formulae.brew.sh does and allows the removal
of the push job from Homebrew/brew.

.github/workflows/scheduled.yml

@@ -0,0 +1,36 @@
+name: Scheduled regeneration
+on:
+  schedule:
+    # Once every other hour
+    - cron:  '30 */2 * * *'
+jobs:
+  generate:
+    if: startsWith( github.repository, 'Homebrew/' )
+    runs-on: ubuntu-latest
+    steps:
+    - name: Set up Git repository
+      uses: actions/checkout@master
+
+    - name: Set up Ruby
+      uses: actions/setup-ruby@v1
+      with:
+        ruby-version: '2.6'
+
+    - name: Install RubyGems
+      run: |
+        gem install bundler -v "~>1"
+        bundle install --jobs 4 --retry 3
+
+    - name: Generate site
+      run: |
+        git clone --depth=1 https://github.com/Homebrew/brew
+        bundle exec rake yard build
+
+        # commit and push generated files
+        git add docs
+
+        if ! git diff --no-patch --exit-code HEAD -- docs; then
+          git commit -m "docs: updates from Homebrew/brew" docs
+          git pull --rebase origin master
+          git push
+        fi

.github/workflows/tests.yml

@@ -13,11 +13,11 @@ jobs:
     - name: Set up Ruby
       uses: actions/setup-ruby@v1
       with:
-        version: '>=2.3'
+        ruby-version: '2.6'
 
     - name: Install RubyGems
       run: |
-        gem install bundler
+        gem install bundler -v "~>1"
         bundle install --jobs 4 --retry 3
 
     - name: Generate site

Gemfile.lock

@@ -249,4 +249,4 @@ DEPENDENCIES
   yard
 
 BUNDLED WITH
-   2.0.1
+   1.17.2