Clarify the `kroki_server` configuration property and give examples in the README

[9ao9ai9ar]

I have been using the configuration option

    server: https://kroki.io/plantuml

without problems until today, when the web service started returning 5xx errors. In my initial search for the cause, I read the updated README of plantuml-markdown and found out that there is also the kroki_server configuration property, and from skimming through the source code it seems the correct way to specify it is

    kroki_server: https://kroki.io

and it will always be picked before the server property if both are set.

However, none of those are evident from the README without consulting the source code, and why should one choose the latter over the former when they both appear to achieve the same results? Also, could there be a fallback mechanism such that if a PlantUML server is returning errors, then a secondary PlantUML server is tried for rendering?