Missing documentation on menu on the Dev Portal configuration files

### Where is the problem?

https://docs.konghq.com/gateway/latest/developer-portal/structure-and-file-types/

### What happened?

We discovered that there is a menu configuration in the portal.cli.conf that is undocumented

### What did you expect to happen?

To find documentation on the `menu` section of the portal.cli.conf

### 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 with the developer portal structure and file types page linked in the issue, then inspect the portal.cli.conf configuration to understand the undocumented menu section. Done means the menu configuration is documented on that page; note that this repository is archived and points contributors to developer.konghq.com.

Written by the indexing model from the issue text.