Add multiple subnets docs

[lzap]

We need to add this to install guides:

https://theforeman.org/plugins/katello/3.11/installation/index.html#multiple-subnets-and-domains

[maximiliankolb]

new link: https://theforeman.org/plugins/katello/nightly/installation/index.html#multiple-subnets-and-domains

[ekohl]

I started proper support in theforeman/puppet-foreman_proxy#705 but never finished that.

[adamlazik1]

I think it would be better if theforeman/puppet-foreman_proxy#705 before we start documenting this in order to avoid instructing users to edit config files directly.

[maximiliankolb]

triage: close due to lack of interest