docs(plugin-config): match the config tab, icon and web-action docs to the code

- PLUGIN_CONFIG_QUICK_START / PLUGIN_CONFIGURATION_TABS /
  PLUGIN_CONFIGURATION_GUIDE: there is no "Reset to Defaults" button (the
  tab has Refresh, Update, Uninstall, Save Configuration); plugin config
  hot-reloads (ConfigService + on_config_change), so no restart; the
  schema is found by the fixed name config_schema.json, not a manifest
  config_schema field; the tab row is "Plugin Manager", not "Plugins";
  forms are server-rendered from /v3/partials/plugin-config/<id>; the
  duration hook is get_display_duration()/display_duration; a class_name
  mismatch raises PluginError; the store requires id, name, class_name and
  display_modes (not version); plugin_system.debug/log_level do not exist
  (use run.py -d / LEDMATRIX_DEBUG). Drop "future" features that shipped.
- PLUGIN_CONFIG_CORE_PROPERTIES: list all of CORE_PLUGIN_PROPERTIES,
  including skin, skin_options and the vegas_* tuning keys.
- PLUGIN_CUSTOM_ICONS: icon is only a Font Awesome class (fallback
  fa-puzzle-piece); emoji/URL icons and getPluginIcon() never existed in
  v3. Note that /api/v3/plugins/installed currently omits icon.
- PLUGIN_WEB_UI_ACTIONS (+ example JSON): success_message, error_message
  and step1_message are never read.

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

docs/PLUGIN_WEB_UI_ACTIONS_EXAMPLE.json

@@ -37,9 +37,6 @@
       "script": "authenticate_spotify.py",
       "oauth_flow": true,
       "section_description": "Authenticate with Spotify or YouTube Music to enable music playback display.",
-      "success_message": "Spotify authentication completed successfully",
-      "error_message": "Spotify authentication failed",
-      "step1_message": "Authorization URL generated",
       "step2_prompt": "Please paste the full redirect URL from Spotify after authorization:",
       "step2_button_text": "Complete Authentication"
     },
@@ -51,9 +48,7 @@
       "button_text": "Authenticate YTM",
       "icon": "fab fa-youtube",
       "color": "red",
-      "script": "authenticate_ytm.py",
-      "success_message": "YouTube Music authentication completed successfully",
-      "error_message": "YouTube Music authentication failed"
+      "script": "authenticate_ytm.py"
     }
   ],
   "versions": [