docs: replace stale API names, line numbers and the api_v3.py path

- ADVANCED_FEATURES: StreamManager methods that exist
  (get_next_segment, take_next_group, refresh, advance_cycle, ...), and the
  real on-demand status envelope ({status, data: {state, service}})
- app.py:199 / :144 / :607-619 line citations and
  web_interface/blueprints/api_v3.py (now a package) replaced with file and
  function names in ADVANCED_FEATURES, CONFIG_DEBUGGING,
  PLUGIN_ARCHITECTURE_SPEC, PLUGIN_QUICK_REFERENCE,
  PLUGIN_CONFIGURATION_TABS, TROUBLESHOOTING and web_interface/README
- CONFIG_DEBUGGING: partial /config/main saves change only sent keys; use
  /config/raw/main to replace the file; describe where validation runs
- TROUBLESHOOTING: clear_cache.py needs --clear-all (no args only prints
  usage)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/TROUBLESHOOTING.md

@@ -273,7 +273,7 @@ sudo systemctl cat ledmatrix-web | grep User
 1. **Verify file structure:**
    ```bash
    ls -l web_interface/app.py
-   ls -l web_interface/blueprints/api_v3.py
+   ls -ld web_interface/blueprints/api_v3/
    ls -l web_interface/blueprints/pages_v3.py
    ```
 
@@ -531,7 +531,7 @@ sudo systemctl cat ledmatrix-web | grep User
 
    ```bash
    # Clear the cache with the helper script
-   sudo python3 scripts/utils/clear_cache.py
+   sudo python3 scripts/utils/clear_cache.py --clear-all
 
    # Or remove files manually from the cache dir in use, e.g.:
    sudo rm -rf /var/cache/ledmatrix/*