Use dependabot to manage GitHub Actions upgrades (#66)

The bot will send PRs to update the versions of the Actions we use.

.github/dependabot.yml

@@ -0,0 +1,12 @@
+# To get started with Dependabot version updates, you'll need to specify which
+# package ecosystems to update and where the package manifests are located.
+# Please see the documentation for all configuration options:
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+
+version: 2
+updates:
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/docs.yml

@@ -128,7 +128,7 @@ jobs:
           path: doc/_build/html
 
       - name: Checkout the gh-pages branch in a separate folder
-        uses: actions/checkout@28c7f3d2b5162b5ddd3dfd9a45aa55eaf396478b
+        uses: actions/checkout@v2
         with:
           ref: gh-pages
           # Checkout to this folder instead of the current one