[DOCS] Describe the role of `use_proxy_proto: true` in Envoy sample config 👺👺👺

Hi there.
In a quest to make Limitador working on OpenShift, I have spent some good hours trying to understand what I was doing wrong in adapting the `kubernetes` example.

It turned out that the source of the issue was `use_proxy_proto: true` !!!

I've searched in your repo and I have found a reference to that in the kind setup script: https://github.com/3scale-labs/limitador/blob/ace7dd773300b8ab76d95a528380dd3cf53a531e/limitador-server/script/kind-setup.sh

My suggestion would be to mention it's role explicitly in the docs with the correct setting required.

FWIW, this is the output from `curl` when the flag is set to `true`:
```bash
curl: (52) Empty reply from server
```

Start with the Kubernetes example and the referenced limitador-server/script/kind-setup.sh file to trace where use_proxy_proto: true is configured. Update the relevant documentation to explain the setting's role and state the correct value for the sample configuration, including the failure symptom described in the issue.

Written by the indexing model from the issue text.