Extend 3scale APIManager documentation to explain replicas configurability

The section of the documentation related to 3scale APIManager deployment via operator should be extended to explain that now the number of replicas of some of the components can be set. This allows you to have "high availability" at replicas level in those components. This was automatically done when setting the highAvailability section with the field set to 'true' but the behavior has changed and it's now controlled more granularly through the replicas fields available in the APIManager CR. Update the documentation accordingly to achieve the same effect.

This comes from the issue THREESCALE-3934. Look at that issue and its comments for more detail.

We should update 2.7 documentation to reflect that and release the 2.8 documentation with that too.