docs(readme): restructure both READMEs and cover every current feature

Contents, highlights and quick start first, then features in the order
a club uses them, then installation, decoders and development. Adds the
start count-down, series hub, practice tooling, audio, OBS and security
features, and removes outdated statements.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.39.0",
+  "version": "1.39.1",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {