chore: Add a bot to update github actions (#197)

# Motivation
CI is emitting warnings about outdated GitHub actions. Updating GitHub
actions is a chore, but Dependabot can reduce the workload by creating
PRs to update actions.

![Screenshot from 2024-02-20
13-40-45](https://github.com/dfinity/stable-structures/assets/5982633/4e28f74f-502b-43cc-8d57-c4f2e8a9429c)

Note: Humans still need to review the updates, so this reduces but does
not remove the maintenance effort.

# Changes
- Added a Dependabot configuration to update GitHub actions
periodically.

# Tests
- I use this same configuration in other repositories, and to date it
has worked as expected.

.github/dependabot.yml

@@ -0,0 +1,9 @@
+# Please see the documentation for all configuration options:
+# https://help.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"