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
cf2fabc40f