docs: archive the custom-icons status report, cross-link config docs, document assets/

PLUGIN_CUSTOM_ICONS_FEATURE.md was a 'What Was Implemented' status
report duplicating the actual guide (PLUGIN_CUSTOM_ICONS.md) — moved to
docs/archive/ per the docs index's own policy. The overlapping
plugin-config docs keep their content but PLUGIN_CONFIG_ARCHITECTURE.md
now states up front which doc is canonical for which purpose.

assets/README.md is new and load-bearing: assets/stocks, weather,
news_logos and broadcast_logos have zero references in this repo's code,
which makes them look deletable — but store plugins (ledmatrix-stocks,
ledmatrix-weather, news, odds-ticker) resolve those exact paths at
runtime against the install directory. The README records that evidence
so a future cleanup doesn't break installed plugins.

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

assets/README.md

@@ -0,0 +1,22 @@
+# assets/
+
+Static assets bundled with LEDMatrix. **Do not delete these directories** —
+several look unused from core code alone but are resolved at runtime by
+installed store plugins.
+
+| Directory | Used by |
+|---|---|
+| `fonts/` | Core (`FontManager`, `DisplayManager`) and most plugins |
+| `sports/` | Core logo tooling (`src/logo_downloader.py`) and the sports scoreboard plugins; team logos are downloaded here on demand |
+| `stocks/` | `ledmatrix-stocks` plugin (`crypto_icons/`, `ticker_icons/`) |
+| `weather/` | `ledmatrix-weather` plugin (weather icons) |
+| `news_logos/` | `news` plugin |
+| `broadcast_logos/` | `news` and `odds-ticker` plugins |
+| `static_images/` | Legacy examples referenced in the `static-image` plugin's docs; the plugin itself stores uploads under `assets/plugins/<plugin-id>/uploads/` |
+| `plugins/` | Per-plugin uploaded files (`assets/plugins/<plugin-id>/uploads/`), served by the web interface |
+
+Plugins resolve these paths relative to the LEDMatrix install directory, so
+the directories are part of the de-facto plugin API even where no file in
+this repo references them. New plugins should bundle their own assets or
+use the per-plugin upload directory instead of adding top-level
+directories here.