Create a separate Clustering changes section

[tsegismont]

Currently, we have Clustered flag removed from options classes and Changes in Infinispan cluster manager in the Miscellaneous Changes section.

We should have a Clustering changes section which contains:

• Clustered flag removed from options classes
• Changes in Infinispan cluster manager
• Changes in Hazelcast cluster manager (content already exists in the wiki)
• Changes in Apache Ignite cluster manager (content already exists in the wiki)
• Changes in Apache Zookeeper cluster manager (content does not exist yet in the wiki - Update the 4.0 deprecation and breaking changes wiki page vertx-zookeeper#110)
• Upgrading clusters gradually