docs: add Customers tab to Reset Budgets section

Add a 'Customers' tab to the Reset Budgets section showing how to use
/customer/new and /customer/update with budget_duration to set up
automatically resetting budgets for customers (end-users).

Co-authored-by: Krrish Dholakia <krrish-berri-2@users.noreply.github.com>

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

@@ -621,6 +621,48 @@ curl 'http://0.0.0.0:4000/team/new' \
   "budget_duration": "30s", # 👈 KEY CHANGE
 }'
 ```
+</TabItem>
+<TabItem value="customers" label="Customers">
+
+Use `/customer/new` to create a customer with a `budget_duration`. The budget will automatically reset at the end of the specified duration.
+
+```bash
+curl 'http://0.0.0.0:4000/customer/new' \
+--header 'Authorization: Bearer <your-master-key>' \
+--header 'Content-Type: application/json' \
+--data-raw '{
+  "user_id": "my-customer-id",
+  "max_budget": 10,
+  "budget_duration": "30d", # 👈 KEY CHANGE
+}'
+```
+
+To update an existing customer's budget duration:
+
+```bash
+curl 'http://0.0.0.0:4000/customer/update' \
+--header 'Authorization: Bearer <your-master-key>' \
+--header 'Content-Type: application/json' \
+--data-raw '{
+  "user_id": "my-customer-id",
+  "max_budget": 10,
+  "budget_duration": "30d", # 👈 KEY CHANGE
+}'
+```
+
+Then pass the customer's `user_id` as the `user` parameter in `/chat/completions` requests:
+
+```bash
+curl 'http://0.0.0.0:4000/chat/completions' \
+--header 'Authorization: Bearer <your-api-key>' \
+--header 'Content-Type: application/json' \
+--data '{
+  "model": "gpt-4o",
+  "user": "my-customer-id",
+  "messages": [{"role": "user", "content": "Hello!"}]
+}'
+```
+
 </TabItem>
 </Tabs>