chore: add Dependabot config for automated dependency updates (#24)

Enables weekly Dependabot updates (minor/patch grouped to limit PR noise)
for the ecosystems present in this repo (GitHub Actions plus the relevant
package managers and Dockerfiles). Config-only, no code changes.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,38 @@
+# Dependabot configuration — https://docs.github.com/code-security/dependabot
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    groups:
+      actions:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    groups:
+      pip:
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"
+
+  - package-ecosystem: "npm"
+    directory: "/docs"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    groups:
+      npm:
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"