Add config option to control child page limit

[viren-nadkarni]

• Adds an undocumented option which controls how many child pages per parent are shown in the sidebar.
• Fixes sidebar_menu_truncate needs to be documented #979

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Preview: https://deploy-preview-1334--docsydocs.netlify.app/docs/adding-content/navigation/#section-menu-options

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[chalin]

LGTM, after making some copy edits.

Thanks all!