Add Fedora Flatpak index for non-Satellite builds (#4395)

This patch adds an example that users can use without authentication.
This helps users to use Foreman+Katello and maintainers to test the docs
:)

Note that "https://quay.io/organization/fedora/" does not work. Pulp
returns an "308 Permanent Redirect" error.

URL comes from Samir via https://community.theforeman.org/t/rfc-flatpak-support-in-katello/39768

Author: maximiliankolb

guides/common/modules/proc_creating-a-flatpak-remote.adoc

@@ -13,7 +13,10 @@ You can create a Flatpak remote to access and manage Flatpak repositories in {Pr
 . Click *Create new*.
 . In the *Name* field, enter a name for the Flatpak remote.
 . In the *URL* field, enter the URL of the Flatpak remote.
-  For example, to use the Red{nbsp}Hat Flatpak index, enter: `https://flatpaks.redhat.io/rhel/`.
+For example, to use the Red{nbsp}Hat Flatpak index, enter: `https://flatpaks.redhat.io/rhel/`.
+ifndef::satellite[]
+To use the Fedora Flatpak index, enter: `https://registry.fedoraproject.org/`.
+endif::[]
 . If the Flatpak remote requires authentication, enter the required credentials.
 For example, synchronizing Red{nbsp}Hat Flatpaks from `registry.redhat.io` requires authentication.
 For more information, see https://access.redhat.com/articles/RegistryAuthentication#creating-registry-service-accounts-6[Creating Registry Service Accounts].