Add ignore directives to the dependabot configuration

Add ignore directives and an attribution comment to the dependabot
configurtation. These will be uncommented in descendants of this
project (cisagov/skeleton-python-library) so that version changes to
these Actions are controlled through this project.

.github/dependabot.yml

@@ -19,6 +19,9 @@ updates:
       - dependency-name: actions/setup-python
       - dependency-name: hashicorp/setup-terraform
       - dependency-name: mxschmitt/action-tmate
+      # # Managed by cisagov/skeleton-python-library
+      # - dependency-name: actions/download-artifact
+      # - dependency-name: actions/upload-artifact
 
   - package-ecosystem: "pip"
     directory: "/"