Adapt SANs doc from MKE 3

[byDimasik]

In MKE 3 docs, we had this page describing how to add SANs to MKE certs - https://docs.mirantis.com/mke/3.7/ops/administer-cluster/add-sans-to-cluster-certs.html

In MKE 4, we need a revamped version of this doc since not all things work the same way.

In MKE 4, there are 2 places where users can set SANs

1. Kubernetes API server serving cert
2. MKE UI TLS cert

To append SANs to the Kube API server, simply add them to the config file

spec:
  apiServer:
    sans:
    - san1.com
    - san2.com

and run mkectl apply.

This will only update the SANs of the Kube API server.

We currently do not support adding SANs to MKE UI TLS certs, therefore the only way to have a desired list of sans is by issuing your own TLS cert for ingress controller and using it as described in this doc - #108