Document the log centralization system

[alexandre-allard]

Component: documentation

Why this is needed:
To explain what's the log centralization system is, how to interact and configure it.

What should be done:
The sizing section in Introduction page is updated to include log centralization service impact.
The sizing rule takes in account the retention period, workloads expected log rate and workload predefined indices.
This rule is to be known by solution developers to properly size the service based on the workload properties.
The Post Installation page is updated to indicate that persistent storage is needed for log centralization service.
A new page should be added to explains how to operate the service and how to forward logs to an external log centralization system.
The Cluster Monitoring page is updated to describe the log centralization service.

Implementation proposal (strongly recommended):

Test plan:

[alexandre-allard]

Already done and sizing rules are to be defined in Solutions documentation because they greatly depend on the applications (still we have a dumb pretty pessimistic rule in our documentation).