Add dependabot configuration file

This commit adds an initial Dependabot configuration to:
* Submit pull requests for security updates and version updates for GH Action runner dependencies.

At a later point in time, we could consider enabling it for Composer dependencies as well.

The configuration has been set up to:
* Run weekly (for now).
* Submit a maximum of 5 pull requests at a time.
    If additional pull requests are needed, these will subsequently be submitted the next time Dependabot runs after one or more of the open pull requests have been merged.
* The commit messages for PRs submitted by Dependabot will be prefixed according the unofficial conventions used in this repo up to now.

Refs:
* https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
* https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#versioning-strategy

.github/dependabot.yml

@@ -0,0 +1,14 @@
+# Dependabot configuration.
+#
+# 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"
+    open-pull-requests-limit: 5
+    commit-message:
+      prefix: "GH Actions:"