Caching service documentation

[pavel-jares-bcm]

This issue is based on #4408

The aim of this issue is simplify and correct the documentation about caching service (especially for the mode using Infinispan).

https://docs.zowe.org/stable/extend/extend-apiml/api-mediation-infinispan/#infinispan-configuration

The documentation said that we need to configure each HA instance differently:

haInstances:
  lpar1:
    components:
      caching-service:
        storage:
          mode: infinispan
          infinispan:
            jgroups.port: 7600
            initialHosts: lpar2[7600]
            
  lpar2:
    components:
      caching-service:
        storage:
          mode: infinispan
          infinispan:
            jgroups.port: 7600
            initialHosts: lpar1[7600]

There is no reason to do that, because JGroup itself suggests to set initialHosts to list of all instances (including the one to configure). The rest of properties are common and could be configured in the components.caching element. For old version is needed to define different storage location, but it is not true for v3 and since #4408 neither for 2.18.4 and later.

For v2 we should also document the requirement for keystore, see #4420