Custom sidebar hides "On this page" table of contents

When users enable a custom sidebar (_sidebar), the auto-generated "On this page" table of contents disappears. Users lose the ability to navigate between headings on the current page.

• User creates custom sidebar
• "On this page" section (showing page headings) is hidden behind the chevron
• Users can no longer jump to sections within the page they're viewing

Move "On this page" to the right side as a collapsible panel:

• Keep custom/default wiki navigation on the left
• Show an outline for the table of contents on the right side
• Make it collapsible as the default and expandable on hover
• Clear visual separation from main content