Fix link in installing server

With external authetication moved to Installing Server, this link also
in Installing Server now becomes xref.

But there are more issues. The assembly is included in three guides:
Installing Server, Installing Server Disconnected, and Installing Proxy.
Installing Server and Installing Server Disconnected have the same
context for satellite. I am opening a separate issue to deal with this
later. For now, I came up with a net of ifevals that works. It is not a
clean solution but I could not come up with any other.

BZ link: https://bugzilla.redhat.com/show_bug.cgi?id=2188809

Author: adamlazik1

guides/common/assembly_configuring-external-idm-dns.adoc

@@ -25,7 +25,18 @@ To revert to internal DNS service, use the following procedure:
 You are not required to use {ProductName} to manage DNS.
 When you are using the realm enrollment feature of {Project}, where provisioned hosts are enrolled automatically to IdM, the `ipa-client-install` script creates DNS records for the client.
 Configuring {ProductName} with external IdM DNS and realm enrollment are mutually exclusive.
-For more information about configuring realm enrollment, see {AdministeringDocURL}External_Authentication_for_Provisioned_Hosts_admin[External Authentication for Provisioned Hosts] in _{AdministeringDocTitle}_.
+For more information about configuring realm enrollment, see
+ifeval::["{context}" == "{project-context}"]
+ifeval::["{mode}" == "connected"]
+xref:External_Authentication_for_Provisioned_Hosts_{context}[].
+endif::[]
+ifeval::["{mode}" == "disconnected"]
+{InstallingServerDocURL}External_Authentication_for_Provisioned_Hosts_{project-context}[External Authentication for Provisioned Hosts] in _{InstallingServerDocTitle}_.
+endif::[]
+endif::[]
+ifeval::["{context}" == "{smart-proxy-context}"]
+{InstallingServerDocURL}External_Authentication_for_Provisioned_Hosts_{project-context}[External Authentication for Provisioned Hosts] in _{InstallingServerDocTitle}_.
+endif::[]
 
 //Configuring Dynamic DNS Update with GSS-TSIG Authentication
 include::modules/proc_configuring-dynamic-dns-update-with-gss-tsig-authentication.adoc[leveloffset=+1]