[ResponseOps] add doc to connectors that can host their own domain names with TLS certificates

[pmuellr]

Some of the connectors we support, for instance ServiceNow, allow customers to upload their own TLS certificates when they set up custom hosts. For example, ServiceNow. When a customer makes use of this facility, they will likely need to add configuration to Kibana to provide the certificate information when the connection is made to the server.

This would typically be done by adding a new entry kibana.yml entry for xpack.actions.customHostSettings, for the specific server url, using the certificateAuthoritiesData property.

We should note this somewhere in the doc for these connectors - perhaps by referring to a single piece of doc explaining this, or a copied-in section would be fine as well. The ServiceNow doc is already kinda busy - feels like a "link" might get lost in the haystack there - having a portable section of code to copy in to relevant connectors is maybe the best.

We'll need to figure out which servers support this sort of thing. For now at least, appears ServiceNow is in that list.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)