Add scala-js-dom docs as submodule

.github/.dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+- package-ecosystem: gitsubmodule
+  directory: "doc/scala-js-dom/"
+  schedule:
+    interval: daily
+  open-pull-requests-limit: 1
+  target-branch: main

.github/workflows/ci.yml

@@ -10,7 +10,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
+        with:
+          submodules: true
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:

.github/workflows/deploy.yml

@@ -7,7 +7,9 @@ jobs:
   deploy:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
+        with:
+          submodules: true
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "doc/scala-js-dom"]
+	path = doc/scala-js-dom
+	url = https://github.com/scala-js/scala-js-dom.git
+	branch = docs