update documentation requirements and dependabot configuration

Change-Id: I652872286c5975d770abb97513469f4527807d7f

.github/dependabot.yml

@@ -10,3 +10,10 @@ updates:
     open-pull-requests-limit: 10
     schedule:
       interval: "daily"
+    target-branch: "master"
+    labels:
+      - "documentation"
+      - "dependencies"
+      - "ci:docs-only"
+    reviewers:
+      - "samjwu"