Add a clustered transactional cache example for Infinispan Server

We should add a transactional cache example as a separate configuration in docs/examples folder.

There's no such example at the moment, and this is really needed in order to avoid WARN messages when conditional versioned operations are used, e.g.

08:38:08,920 WARN  [org.infinispan.server.hotrod.Decoder2x$] (HotRodServerWorker-6-8) ISPN006010: 
Conditional operation 'ReplaceIfUnmodifiedRequest' should be used with transactional caches, otherwise data inconsistency issues could arise under failure situations

Also, this WARN message should only appear when the cache is clustered and there's > 1 node in the cluster, but currently it also appears in local caches, but this is a different issue...