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 (flow-framed, not a feature checklist), Example flow, Running (requirements as a list, then the command, then what it loads), Testing (framed as a pattern worth copying), OpenID Connect (its own section), and Reference (technologies, build config). Cuts the OAuth-101 explanation and the code-to-test navigation table, refreshes the OIDC example, and drops the dev task list.

Author: ardetrick