docs: complete the docs index and refresh the web interface file tree

docs/README.md's own policy says every page must be linked from the
index, yet five weren't — including the entire skin system
(SKIN_SYSTEM.md, CREATING_SKINS.md), ADAPTIVE_LAYOUT.md,
plugin-safety-harness.md and SPORTS_UNIFICATION.md. Each is now listed
in the section it belongs to, and PLUGIN_ARCHITECTURE_SPEC.md is marked
historical in the index (the doc itself already carries the banner).
web_interface/README.md's static/v3 tree showed only app.css/app.js;
it now reflects the actual contents.

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

web_interface/README.md

@@ -30,7 +30,12 @@ web_interface/
 └── static/                   # CSS/JS assets
     └── v3/
         ├── app.css
-        └── app.js
+        ├── app.js
+        ├── manifest.json     # PWA manifest
+        ├── plugins_manager.js
+        ├── icons/            # PWA / touch icons
+        ├── js/               # Alpine, htmx, app shell, widgets, utils
+        └── vendor/           # codemirror, fontawesome
 ```
 
 ## Running the Web Interface