docs should emphasize simplicity of schema-registry service

Quoting @ewencp,
"I think the bigger takeaway here is that we need to be clear in our documentation that the schema registry should be extremely low maintenance. Instead of thinking "oh man, now I have to maintain 3 important services instead of 2", it should be "I just have to keep a couple of instances of this running and I get all these awesome benefits of using schemas". Maybe there are some changes in the documentation that we can make to make it very clear that high availability for this service should just be a matter of keeping a couple of processes running."

The issue does not name a documentation file or test. Begin by locating the schema-registry service's deployment and high-availability documentation; done means those docs clearly present the service as low-maintenance and explain that high availability comes from running a couple of instances.

Written by the indexing model from the issue text.