Geo: Provide guidance for Geo site names

Follow up from #330024 (comment 570144757):

it is preferable not to tie a geo_node_name to a domain name, since e.g. when performing a Geo site failover, domain names are usually pointed to different Geo sites. A better choice is the physical location, e.g. gitlab-usa, or even a totally arbitrary name.

We should add this guidance to Geo documentation. At the moment, we give no recommendation on Geo site naming.