docs: refresh and correct stale documentation across repo

Walked the README and docs/ tree against current code and fixed several
real bugs and many stale references. Highlights:

User-facing
- README.md: web interface install instructions referenced
  install_web_service.sh at the repo root, but it actually lives at
  scripts/install/install_web_service.sh.
- docs/GETTING_STARTED.md: every web UI port reference said 5050, but
  the real server in web_interface/start.py:123 binds 5000. Same bug
  was duplicated in docs/TROUBLESHOOTING.md (17 occurrences). Fixed
  both.
- docs/GETTING_STARTED.md: rewrote tab-by-tab instructions. The doc
  referenced "Plugin Store", "Plugin Management", "Sports Configuration",
  "Durations", and "Font Management" tabs - none of which exist. Real
  tabs (verified in web_interface/templates/v3/base.html) are: Overview,
  General, WiFi, Schedule, Display, Config Editor, Fonts, Logs, Cache,
  Operation History, Plugin Manager (+ per-plugin tabs).
- docs/GETTING_STARTED.md: removed references to a "Test Display"
  button (doesn't exist) and "Show Now" / "Stop" plugin buttons. Real
  controls are "Run On-Demand" / "Stop On-Demand" inside each plugin's
  tab (partials/plugin_config.html:792).
- docs/TROUBLESHOOTING.md: removed dead reference to
  troubleshoot_weather.sh (doesn't exist anywhere in the repo); weather
  is now a plugin in ledmatrix-plugins.

Developer-facing
- docs/PLUGIN_API_REFERENCE.md: documented draw_image() doesn't exist
  on DisplayManager. Real plugins paste onto display_manager.image
  directly (verified in src/base_classes/{baseball,basketball,football,
  hockey}.py). Replaced with the canonical pattern.
- docs/PLUGIN_API_REFERENCE.md: documented cache_manager.delete() doesn't
  exist. Real method is clear_cache(key=None). Updated the section.
- docs/PLUGIN_API_REFERENCE.md: added 10 missing BasePlugin methods that
  the doc never mentioned: dynamic-duration hooks, live-priority hooks,
  and the full Vegas-mode interface.
- docs/PLUGIN_DEVELOPMENT_GUIDE.md: same draw_image fix.
- docs/DEVELOPMENT.md: corrected the "Plugin Submodules" section. Plugins
  are NOT git submodules - .gitmodules only contains
  rpi-rgb-led-matrix-master. Plugins are installed at runtime into the
  plugins directory configured by plugin_system.plugins_directory
  (default plugin-repos/). Both internal links in this doc were also
  broken (missing relative path adjustment).
- docs/HOW_TO_RUN_TESTS.md: removed pytest-timeout from install line
  (not in requirements.txt) and corrected the test/integration/ path
  (real integration tests are at test/web_interface/integration/).
  Replaced the fictional file structure diagram with the real one.
- docs/EMULATOR_SETUP_GUIDE.md: clone URL was a placeholder; default
  pixel_size was documented as 16 but emulator_config.json ships with 5.

Index
- docs/README.md: rewrote. Old index claimed "16-17 files after
  consolidation" but docs/ actually has 38 .md files. Four were missing
  from the index entirely (CONFIG_DEBUGGING, DEV_PREVIEW,
  PLUGIN_ERROR_HANDLING, STARLARK_APPS_GUIDE). Trimmed the navel-gazing
  consolidation/statistics sections.

Out of scope but worth flagging:
- src/plugin_system/resource_monitor.py:343 and src/common/api_helper.py:287
  call cache_manager.delete(key) but no such method exists on
  CacheManager. Both call sites would AttributeError at runtime if hit.
  Not fixed in this docs PR - either add a delete() shim or convert
  callers to clear_cache().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/DEVELOPMENT.md

@@ -141,19 +141,27 @@ stage('Checkout') {
 
 ---
 
-## Plugin Submodules
+## Plugins
 
-Plugin submodules are located in the `plugins/` directory and are managed similarly:
+Plugins are **not** git submodules of this repository. The plugins
+directory (configured by `plugin_system.plugins_directory` in
+`config/config.json`, default `plugin-repos/`) is populated at install
+time by the plugin loader as users install plugins from the Plugin Store
+or from a GitHub URL via the web interface. Plugin source lives in a
+separate repository:
+[ChuckBuilds/ledmatrix-plugins](https://github.com/ChuckBuilds/ledmatrix-plugins).
 
-**Initialize all plugin submodules:**
-```bash
-git submodule update --init --recursive plugins/
-```
+To work on a plugin locally without going through the Plugin Store, clone
+that repo and symlink (or copy) the plugin directory into your configured
+plugins directory — by default `plugin-repos/<plugin-id>/`. The plugin
+loader will pick it up on the next display restart. The directory name
+must match the plugin's `id` in `manifest.json`.
 
-**Initialize a specific plugin:**
-```bash
-git submodule update --init --recursive plugins/hockey-scoreboard
-```
-
-For more information about plugins, see the [Plugin Development Guide](.cursor/plugins_guide.md) and [Plugin Architecture Specification](docs/PLUGIN_ARCHITECTURE_SPEC.md).
+For more information, see:
 
+- [PLUGIN_DEVELOPMENT_GUIDE.md](PLUGIN_DEVELOPMENT_GUIDE.md) — end-to-end
+  plugin development workflow
+- [PLUGIN_ARCHITECTURE_SPEC.md](PLUGIN_ARCHITECTURE_SPEC.md) — plugin system
+  specification
+- [DEV_PREVIEW.md](DEV_PREVIEW.md) — preview plugins on a desktop without a
+  Pi