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

docs/PLUGIN_CONFIG_ARCHITECTURE.md

@@ -1,5 +1,11 @@
 # Plugin Configuration Tabs - Architecture
 
+> This page covers internals (how the config system works under the
+> hood). For designing a plugin's config schema, the canonical guide is
+> [PLUGIN_CONFIGURATION_GUIDE.md](PLUGIN_CONFIGURATION_GUIDE.md); for
+> the user-facing tabs feature, see
+> [PLUGIN_CONFIGURATION_TABS.md](PLUGIN_CONFIGURATION_TABS.md).
+
 ## System Architecture
 
 ### Component Overview