docs(dashboard): document page-scoped plugin slots (#15662)

Follow-up to PR #15658. The feature PR introduced page-scoped slots
(<page>:top / <page>:bottom inside every built-in page) but only
touched the Shell slots catalogue. Adds proper narrative coverage so
plugin authors find the feature.

Changes
- extending-the-dashboard.md:
  - Frontmatter description + intro bullet now mention page-scoped slots
  - New TOC entry "Augmenting built-in pages (page-scoped slots)"
  - New dedicated subsection after "Replacing built-in pages"
    explaining the heavy-vs-light tradeoff, listing the pages that
    expose slots, and showing a worked manifest + IIFE example with
    tab.hidden: true
  - Cross-link from the tab.override section pointing readers to the
    lighter augmentation option
- web-dashboard.md:
  - Bullet mentioning "page-scoped slots (inject widgets into
    built-in pages without overriding them)"

Validation
- TOC anchor "#augmenting-built-in-pages-page-scoped-slots" matches
  the generated heading slug
- Code fences balanced (64, even)
- Pre-existing docusaurus build errors (skills.json, api-server.md
  link) reproduce on bare main -- not introduced here

website/docs/user-guide/features/web-dashboard.md

@@ -342,6 +342,6 @@ To build your own theme, add a plugin tab, inject into shell slots, or expose pl
 
 - Theme YAML schema — palette, typography, layout, assets, componentStyles, colorOverrides, customCSS
 - Layout variants — `standard`, `cockpit`, `tiled`
-- Plugin manifest, SDK, shell slots, backend FastAPI routes
+- Plugin manifest, SDK, shell slots, page-scoped slots (inject widgets into built-in pages without overriding them), backend FastAPI routes
 - A full combined theme-plus-plugin walkthrough (Strike Freedom cockpit demo)
 - Discovery, reload, and troubleshooting