Installation instructions fail on Fedora - rpm --eval '%{rhel}' returns empty

### Where is the problem?

https://developer.konghq.com/gateway/install/#linux

### What happened?

Problem
The Linux installation instructions at https://developer.konghq.com/gateway/install/#linux fail on Fedora.

The repo setup command uses:
$(rpm --eval '%{rhel}')
This returns an empty value on Fedora, causing the yum repo to be misconfigured.

## Error
No match for argument: kong-enterprise-edition-3.13.0.0
Error: Unable to find a match: kong-enterprise-edition-3.13.0.0

## Solution
Manually specifying `codename=9` in the URL works:
curl -1sLf "https://packages.konghq.com/public/gateway-313/config.rpm.txt?distro=el&codename=9" | sudo tee /etc/yum.repos.d/kong-gateway-313.repo

### What did you expect to happen?

Add a note for Fedora users to use `codename=9` instead of `$(rpm --eval '%{rhel}')`.

### 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 Linux installation section of the linked Kong Gateway installation page and verify how the repository command behaves on Fedora. Add the requested Fedora note using the working codename=9 form, then confirm the instructions produce a usable repository configuration; note that this repository is archived and points to a newer documentation repository.

Written by the indexing model from the issue text.