docs(cost_tracking.md): add docs for cost tracking dall e 3 calls

docs/my-website/docs/proxy/cost_tracking.md

@@ -0,0 +1,18 @@
+# Cost Tracking - Azure
+
+Set base model for cost tracking azure image-gen call
+
+## Image Generation 
+
+```yaml
+model_list: 
+  - model_name: dall-e-3
+    litellm_params:
+        model: azure/dall-e-3-test
+        api_version: 2023-06-01-preview
+        api_base: https://openai-gpt-4-test-v-1.openai.azure.com/
+        api_key: os.environ/AZURE_API_KEY
+        base_model: dall-e-3 # 👈 set dall-e-3 as base model
+    model_info:
+        mode: image_generation
+```