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features, and removes outdated statements.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-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "rc-racecore",
-  "version": "1.39.0",
+  "version": "1.39.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "rc-racecore",
-      "version": "1.39.0",
+      "version": "1.39.1",
       "hasInstallScript": true,
       "dependencies": {
         "better-sqlite3": "^12.11.1",