[#65881616] Load balancer documentation

This is ready for review. I've split the documentation into 4 'sections':

* General Load Balancer Service overview
* An example of a 'normal' setup
* More detail on Pool healthchecks (the complicated bit about pools)
* More detail on Virtual Server persistence (the complicated bit about virtual servers)

... with the idea being that the most important information is at the top, working down into a valid setup for most people, then onto the specifics...