Log SteamVR-started instances to logs/<exe>.log

The manifest passes --log-file, so a settings error (CFG-2) ends up in a
log file in the install folder. Package README and docs updated.

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

resources/manifest.vrmanifest.in

@@ -4,6 +4,7 @@
       "app_key": "@VRDASH_APP_KEY@",
       "launch_type": "binary",
       "@VRDASH_MANIFEST_BINARY_KEY@": "../../@CMAKE_INSTALL_BINDIR@/@VRDASH_EXECUTABLE_NAME@",
+      "arguments": "--log-file logs/@VRDASH_EXECUTABLE_NAME@.log",
       "is_dashboard_overlay": true,
       "strings": {
         "en_us": {