ci: add a custom dependabot file (#1960)

This way the requirements files in the examples and tests are not
managed, which should reduce the PR noise.

.github/dependabot.yml

diff --git a/.github/dependabot.yml b/.github/dependabot.yml
new file mode 100644
index 0000000..9632f4e
--- /dev/null
+++ b/.github/dependabot.yml
@@ -0,0 +1,17 @@
+---
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "pip"
+    directories:
+      # Maintain dependencies for our tools
+      - "/docs/sphinx"
+      - "/tools/publish"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 3