docs(reference-app): restructure the README to lead with seeing and running it

Reworks the README onto a community-standard spine (Standard Readme / Make a README): badges + one-line description, what this is, Example flow, Running (requirements, command, then what it loads), Testing (framed as a pattern worth copying), OpenID Connect, and Reference. Cuts the OAuth-101 explanation and the code-to-test navigation table, refreshes the OIDC example, drops the dev task list, and corrects the docker run example to the Hydra version the project actually uses.

Author: ardetrick