category mapping for kibana_sample_data_ecommerce (#415)

Fixes `Breakdown by category` panel in the example dashboard. The result
is incorrect due to our lack of array support for now, but at least it
displays values and not an error.

<img width="851" alt="Screenshot 2024-06-27 at 08 05 50"
src="https://github.com/QuesmaOrg/quesma/assets/2182533/7bbd75cf-56d2-481d-ba63-46c78332940c">

docker/quesma/config/local-dev.yaml

@@ -20,6 +20,7 @@ indexes:
     enabled: true
     mappings:
       products.manufacturer: "text"
+      category: "text"
   kibana_sample_data_flights:
     enabled: true
     mappings: