add dependabot

While reviewing the code I found a lot of reported outdated npm packages
with open security vulnerabilities. Given the sensitive nature of this
projects, it's therefore recommended to keep the project dependencies
up-to-date automatically.

.github/dependabot.yml

@@ -0,0 +1,14 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+  - package-ecosystem: "npm"
+    directory: "/web-admin"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"