NICs filtering and optimization to Admin guide

[Lennonka]

Facts and NIC filtering

https://theforeman-foreman-documentation-preview-pr-3922.surge.sh/nightly/Provisioning_Hosts/index-foreman-el.html#facts-and-nic-filtering_provisioning

Why is this in the provisioning guide? IMHO this belongs in the administration guide.

Optimizing performance by removing NICs from database

https://theforeman-foreman-documentation-preview-pr-3922.surge.sh/nightly/Provisioning_Hosts/index-foreman-el.html#facts-and-nic-filtering_provisioning

Same as Facts and NIC filtering above: belongs in the admin guide.

Originally posted by @ekohl in #3922 (review)