[Docs] Update README to minimize time from clone to running Fleet Server

[kpollich]

Fleet Server's current README includes a lot of context related to compatibility, Elastic Agent, etc before diving into the steps needed to actually run Fleet Server.

We should adjust the README to minimize the time from cloning the repo to getting up and running, optimizing for new users onboarding to the project.

This means we should include

• Golang requirements, setup instructions
• Quick copy/paste commands to run Fleet Server in standalone mode over plain HTTP
• A sanity check to verify Fleet Server is running properly (e.g. check the status API)