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 with a features list, Example flow (the walkthrough), Running (with your own Hydra as a subsection), Testing, and Reference. Cuts the OAuth-101 explanation and the code-to-test navigation table, folds prerequisites into Running, refreshes the OIDC id_token/JWKS example, and drops the dev task list.

Author: ardetrick