This commit adds documentation for configuring Kerberos realm.
Configuring Kerberos realm documentation highlights important
terminology and requirements before creating Kerberos realm.
Most of the documentation is centered around configuration from
Elasticsearch rather than go deep into Kerberos implementation.
Kerberos realm settings are mentioned in the security settings
for Kerberos realm.
With this commit we remove the documentation for the setting
`xpack.monitoring.collection.indices.stats.timeout` which has already
been removed in code.
Closes#32133
Relates #32229
