Add example for routing based on model field in POST payload

Sometimes users need to route based on a value from a POST payload. A workaround is to use a global pre-function in the **rewrite** phase to read the relevant content from the payload and add it as a header, then use header-based routing in the [route definition](https://developer.konghq.com/gateway/entities/route/).

Here’s an example that extracts the **AI model** to invoke based on a JSON field in the payload:

```lua
if not kong.request.get_header("X-Model") then
local b = kong.request.get_body("application/json")
if b and b.model then
kong.service.request.set_header("X-Model", b.model)
end
end
````

We must note that users should be careful with this pattern. Parsing/transforming payloads can be time-consuming. While small payloads have minimal cost, a malicious actor could send a very large payload to create memory pressure on Kong.

We should also recommend pairing this approach with the **Request Size Limiting** plugin.

Start from the linked route definition documentation and identify the appropriate routing example section. Add the Lua example for extracting the model from a JSON POST payload, explain the rewrite-phase and header-routing pattern, and document the payload-size risk with a recommendation to use the Request Size Limiting plugin.

Written by the indexing model from the issue text.