Add dependabot config for updating documentation requirements (#1018)

.github/dependabot.yml

@@ -9,3 +9,9 @@ updates:
     directory: "/" # Location of package manifests
     schedule:
       interval: "weekly"
+  - package-ecosystem: "pip" # See documentation for possible values
+    directory: "/docs/sphinx" # Location of package manifests
+    open-pull-requests-limit: 10
+    schedule:
+      interval: "daily"
+    versioning-strategy: increase