Add support for "self" sidebar roots

[chalin]

Contributes to

• Sidenav support for section-as-root in the sidebar #2328

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Previews

Sidenav root at self:

• https://deploy-preview-2341--docsydocs.netlify.app/docs/adding-content/

Sidenav root for children:

• Root: https://deploy-preview-2341--docsydocs.netlify.app/docs/best-practices/, which still shows the full docs sidenav
• Example of a child: https://deploy-preview-2341--docsydocs.netlify.app/docs/best-practices/site-guidance/, whose sidenav is rooted at it's parent "Best practices" page

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Screenshots

Sidenav root at self:

Sidenav root for children:

Best practices as root for child pages:

Child page example: