Docs: add notification centre documentation pages

[twjeffery]

The Work Side Notification Panel component is shipped (React + Angular wrappers in main lib) but has no documentation on the design system website.

Added in PR #3371 and moved from experimental to main wrappers in PR #3690.

Work needed

• Create MDX file for the panel (work-side-notification-panel.mdx) as a visible component page
• Create MDX file for the item (work-side-notification-item.mdx) as a hidden subcomponent (hidden: true, subcomponent: true), so its API shows on the Panel page (same pattern as Tab under Tabs)
• Run API extraction on the Svelte source to generate JSON files in docs/generated/component-apis/
• Create configuration examples in docs/src/data/configurations/ showing common variants (read/unread states, urgent, panel with multiple items, active tab switching)
• Add relatedComponents linking to work-side-menu since the notification panel is used alongside it
• Add the component to the search index (rebuild with npm run build:search-index)
• Add component thumbnail SVG for the components grid (coordinate with Olu or create placeholder)