docs: correct semantically stale content across the user and developer guides

A second-pass content audit checked the guides' substantive claims
against the code (the first pass only fixed mechanical drift). Fixes:

- GETTING_STARTED: described booting a prebuilt SD image and seeing
  default clock/weather plugins — neither exists. Now documents the real
  install (Pi OS Lite + one-shot installer / first_time_install.sh) and
  that displays come from the Plugin Store. Duration and ordering
  instructions moved to the Rotation tab where the controls actually
  live.
- WEB_INTERFACE_GUIDE: three whole tabs were undocumented (Rotation,
  Backup & Restore, Tools) and the Display tab's Vegas Scroll section
  was unmentioned. Fonts overrides are per display element (not per
  plugin); Logs has an Auto-scroll checkbox (not a Pause button); the
  aspirational keyboard-shortcut list and no-JS claim removed.
- TROUBLESHOOTING: the hand-written service-file template (wrong user,
  wrong ExecStart, dropped the autostart gate) replaced with the real
  systemd/ units + install scripts; recovery steps no longer copy
  placeholder units verbatim; WiFi curl endpoint corrected to /api/v3/;
  cache-clearing advice now targets the real cache locations.
- ADVANCED_FEATURES: removed a false claim that CacheManager has no
  delete(); fixed two example snippets that raise TypeError
  (BackgroundDataService and get_config_file_mode signatures); fixed
  cache paths, a 5-minute TTL that is actually 1 hour, and the vegas
  table now links the complete 26-key reference.
- EMULATOR_SETUP_GUIDE: documented run.py flags that don't exist
  (--plugin/--test-plugins) removed in favor of dev_server.py and
  check_plugin.py; shipped emulator config values corrected (browser
  adapter default on :8888, not pygame).
- PLUGIN_QUICK_REFERENCE: drag-and-drop reordering is shipped, not
  'not yet supported'; discovery-fallback and registry-repo claims
  corrected. PLUGIN_API_REFERENCE: get_vegas_segment_width returns
  panels, not pixels. CONTRIBUTING: the repo uses flake8/mypy/bandit
  pre-commit hooks, not black/ruff, and tests need requirements-test.txt.
- SKIN_SYSTEM/DEVELOPER_QUICK_REFERENCE: stale module paths.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

docs/SKIN_SYSTEM.md

@@ -33,7 +33,7 @@ crashing) simply restores the built-in look.
 ## The render funnel
 
 Every sports scoreboard (baseball, football, basketball, hockey — anything
-built on `src/base_classes/sports.py`) renders through exactly one seam:
+built on the `src/base_classes/sports/` package, `core.py`) renders through exactly one seam:
 `SportsCore._render_game(game, force_clear)`.
 
 1. The mode class's `display()` (live, `SportsUpcoming`, `SportsRecent`)