Private Repository Set up instruction is not clear whether flat repository is allowed or not for the Fleet Server setup

## Description

Per the documentation guide - https://www.elastic.co/guide/en/fleet/current/air-gapped.html#air-gapped-diy-epr - the private repository can be set up for an air-gapped environment so the packages can be downloaded upon fleet server setup.

However, this line of instruction
>If routing traffic through a proxy server is not an option, you can host your own Elastic Package Registry.
>
>The Elastic Package Registry can be deployed and hosted on-site using one of the available Docker images. These docker images include the Elastic Package Registry and a selection of packages.

makes the users interpret that setting up a private repository like jfrog or any other flat server repository, using the configurations like below, can work with the Fleet setup, which does not turn out to be true.

```
xpack.fleet.registryUrl: "http://privivate.custom.repo.com/project/elasticsearch/fleet/"
```

If setting up a flat repository is not supported (as opposed to docker image hosting the repo as a web service), it will be helpful for the users if the documentation clearly indicates that it is either not supported or to suggest some workarounds to setup the flat repository to avoid any confusion.