Improve documentation on Eureka server setup

There were two things in the documentation that puzzled me:

1. Seeing the Client docs first. You can't really get that to work without a server running, so I guess it'd make more sense from a narration point of view to start with the server docs.
2. The server docs create the impression that the very first code sample is all you need as it concludes with: "The server has a home page with a UI, and HTTP API endpoints per the normal Eureka functionality under /eureka/*." Also, it feels a bit thrown together: "Example eureka server". "Eureka background reading:"

What if we started with an introductory paragraph that explains the modes of operation and then gives a quick start with what's needed for a single instance and then concludes with more elaborate discussion about why that's not a good idea in prod etc.

Start with the linked server documentation and compare its placement with the Client documentation. Add introductory guidance on the operating modes, provide a single-instance quick start before the broader discussion, and clarify the production caveats and surrounding headings; done means the setup flow is understandable without the current ordering confusion.