docs: consolidate dashboard themes and plugins into Extending the Dashboard (#15530)

The web-dashboard.md and dashboard-plugins.md pages had overlapping,
partial coverage of the theme and plugin systems. Themes were split
across two pages; the plugin docs had a minimal manifest reference but
no step-by-step guide, no slot catalog, and no theme+plugin demo.

New: user-guide/features/extending-the-dashboard.md — single navigable
reference for all three extension layers (themes, UI plugins, backend
plugins). Includes:

- Theme quick-start + full schema (palette, typography, layout, layout
  variants, assets, componentStyles, colorOverrides, customCSS)
- Plugin quick-start + full schema (manifest, SDK, slots, tab.override,
  tab.hidden, backend routes, custom CSS)
- 10-slot shell catalog with locations
- Plugin discovery + load lifecycle
- Combined theme+plugin walkthrough (Strike Freedom cockpit demo)
- API reference + troubleshooting

web-dashboard.md: trimmed to core tool docs (pages, REST API, CORS,
development). Theme/plugin content now points to the new page with a
built-in themes summary table.

dashboard-plugins.md: deleted (merged into extending-the-dashboard.md).

sidebars.ts: swap 'dashboard-plugins' → 'extending-the-dashboard' under
the Management group.

No user-facing behavior change; docs-only.

website/sidebars.ts

@@ -81,7 +81,7 @@ const sidebars: SidebarsConfig = {
           label: 'Management',
           items: [
             'user-guide/features/web-dashboard',
-            'user-guide/features/dashboard-plugins',
+            'user-guide/features/extending-the-dashboard',
           ],
         },
         {