feat: add "root" section type for flat sidebar grouping without parent title

[zrosenbauer]

Summary

Add a new section property (e.g. root: true or a new top-level pages config) that creates
an isolated sidebar scope where the parent section title is excluded from the sidebar hierarchy
and its child sections are promoted to top-level sidebar items.

Current state

Today, standalone: true isolates a section into its own sidebar scope
(packages/core/src/sync/index.ts:380–408, runtime filtering in
packages/ui/src/theme/components/sidebar/sidebar-filter.ts:31–83), but the section title
still appears as the top-level group in both the sidebar and nav dropdown.

For example, a "Reference" section with standalone: true and children "API" and "CLI" renders as:

Reference          ← nav dropdown
  └─ API           ← sidebar
  └─ CLI

There's no way to make "API" and "CLI" appear as top-level sidebar items within the scope,
hiding "Reference" from the sidebar tree entirely.

Proposed scope

Introduce a root (or similar) property on Section that:

1. Isolates the scope — same as standalone, the section gets its own sidebar context
   (scopes.json, resolveScopedSidebar())
2. Promotes children — child sections are rendered as top-level sidebar items (no wrapping
   parent group in the _meta.json output)
3. Hides the parent title from sidebar — the section title is used only for nav/routing,
   not as a sidebar heading

Affected areas

Area	File	Change
Config type	packages/config/src/types.ts:244–267	Add root?: boolean to Section
Config schema	packages/config/src/schema.ts:99–121	Add root to Zod schema
Resolved type	packages/core/src/sync/types.ts:115–150	Propagate root on ResolvedEntry
Meta generation	packages/core/src/sync/sidebar/meta.ts:80–339	Promote children when root is set
Scope collection	packages/core/src/sync/index.ts:380–408	Treat root sections as standalone scopes
Nav generation	packages/core/src/sync/sidebar/index.ts:15–32	Decide nav behavior for root sections

Example config

{
  title: 'Reference',
  path: '/references',
  root: true,  // children promoted to top-level sidebar items
  items: [
    { title: 'API', path: '/references/api', include: 'docs/references/api/**' },
    { title: 'CLI', path: '/references/cli', include: 'docs/references/cli/**' },
  ],
}

Result: Navigating to /references/api shows a sidebar with "API" and "CLI" as top-level
headings — no "Reference" wrapper.

Open questions

• Naming: root: true, flat: true, or a distinct top-level config key like pages?
• Should the nav item for a root section link to the first child, or show a dropdown?
• Should root imply standalone automatically, or be independent?