docs(fonts): point plugins at plugin_manager.font_manager; drop removed overrides UI

FONT_MANAGER.md told plugins to read display_manager.font_manager, which
does not exist, so a plugin following it failed to load with
AttributeError. The shared FontManager lives on the PluginManager and
BasePlugin._get_font_manager() returns it (with a fallback for harnesses).

Also removes the Fonts-tab override workflow and element-override panels
that #566 deleted, from FONT_MANAGER.md and WEB_INTERFACE_GUIDE.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/WEB_INTERFACE_GUIDE.md

@@ -206,11 +206,11 @@ Manage fonts for your display:
 - See font previews
 - Check font sizes and styles
 
-**Font Overrides:**
-- Overrides are set per display *element* (e.g. a specific score or
-  clock text element), not per plugin
-- Override default font choices for individual elements
-- Preview font changes
+**Font Preview:**
+- Render sample text in any TTF/OTF font at a chosen size
+
+Fonts used by a plugin are chosen in that plugin's own settings tab; the
+Fonts tab has no per-element override editor.
 
 **Delete Fonts:**
 - Remove unused fonts
@@ -327,9 +327,8 @@ The web interface is built on a REST API that you can access programmatically:
 http://your-pi-ip:5000/api/v3
 ```
 
-The API blueprint mounts at `/api/v3` (see
-`web_interface/app.py:199`). All endpoints below are relative to that
-base.
+The API blueprint (`web_interface/blueprints/api_v3/`) is registered at
+`/api/v3` in `web_interface/app.py`.
 
 **Common Endpoints:**
 - `GET /api/v3/config/main` — Get main configuration