Add developer portal /files POST example

### Where is the problem?

https://docs.konghq.com/gateway/latest/developer-portal/portal-api/#/

### What happened?

Hi,

Noticed there isn't currently a POST example for the /files endpoint for the scenario of adding an OpenAPI specification to the portal.

I have been able to do this with the following cul command -

curl -X POST localhost:8001/files -F "path=specs/tfl_openapi.yaml" -F "contents=@tfl_openapi.yaml"

Thanks

Mark

### What did you expect to happen?

A POST example next to https://docs.konghq.com/gateway/latest/developer-portal/portal-api/#/operations/list-files

### Code of Conduct and Community Expectations

- [X] I agree to follow this project's Code of Conduct
- [X] I agree to abide by the Community Expectations

Start at the developer portal Portal API documentation linked in the issue and locate the list-files operation. Add a POST example for uploading an OpenAPI specification using the supplied curl command; done means the example appears next to that operation in the published documentation.

Written by the indexing model from the issue text.